Okay, while I do like some aspects of this, a lot of this seems like unnecessary effort. I'll just list what I have to say in order of the NPCs presented in your post.
The Guide should spawn in at the beginning as he already does. 3 minutes into the game, I'll most likely be passing through the corruption or jungle. If he spawns next to me, he'll probably die. Of course, the Guide helps beginners mostly. But that only supports the fact he should spawn in the beginning. What would the beginners do for the first three minutes? If the Guide was already there, he could get them to build a house much sooner. The Voodoo doll should remain the same. I don't see a reason it needs to be changed. It can teach a new player how the doll works, and if they haven't thrown it into lava already, it might give them that idea, especially if the Guide tells them about the "living sacrifice". It also provides an easy way to kill Andrew for his hat. While it's not many reasons to have it, there's not many reasons to remove it, either.
I shouldn't have to build a bigger house for the Merchant. From a beginner's perspective, they would build a house for the merchant, the merchant would move in, and then tell them to build a bigger house. I like all my NPC houses to be the same, because it bugs me if they aren't. I think the merchant's stock should be 2 stacks by default and 4 stacks during Hardmode ('cause, you know, magic quiver.), if not infinite. Most of these items are extremely easy to obtain, so there's no need to be strict on their limit. As for the other vendors, they should still buy everything, but they would pay less than the Merchant (10% instead of 20%) if the item is not related to them. If you want some realism behind that, think of it like the NPC is paying less for an item and selling it to the Merchant later for profit. Also, this wasn't made clear in your post, but I think their money supply should increase if you buy something.
The Nurse should remain unchanged, as she is important throughout the entire game and doesn't need the extra attention. Plus, I find the quest just plain tedious. Its only purpose is to make the game slightly more "realistic", but I don't think that's necessary. If I can carry thousands of bombs, she can carry a bag. Everyone gets hammerspace! While the random buff idea seems nice, that kind of decreases the demand for potions, making the Wizard's ingredients less valuable, and it can easily be exploited with intentional damage. Besides, the Dryad provides a defense buff anyways, so I keep her near the Nurse as it is.
I like the Dye Trader's changes. It provides a use for an extra magic mirror (No matter how many friends I'm playing with, I always find at least on extra) as well as reversing those multicolored dyes to get just the right color for me when I'm designing an outfit.
I don't really like the Demolitionist's changes. The ability to blow things up is complicated and easily exploited if you think about it. How will these areas be marked? What blocks can be destroyed? As it is, I do not believe there is a way for the game to tell the difference between player-placed blocks and natural blocks. What if some jerk on a server hired a demolitionist to blow up your house? There was a reason Clowns were nerfed. The contract failing seems tedious. The Demolitionist's stock should just increase when the Merchant is alive. At most, an interaction with the Demolitionist followed by an interaction with the Merchant should be required.
I don't think he should sell ores. However, I do think he should sell silt (and possibly fossils in hardmode?) to be processed in an Extractinator.
I think the Dryad's changes should be modified as well. I need nature blocks? Okay, sure. *Plops 10 Living Wood on her ceiling* Done! Her buff should be affected by the balance of H/C/P in the world. This should also reflect her judgement of the world. If at least 20% of the world is C, and there is more C than Hallow, her power will begin to weaken. She will grow weaker the more C is in the world. Hallow will negate some of the negative effects. If the world is in balance, her power will return to normal. If the world is at least 15% Hallowed, and there is more Hallow than C, the player will also receive a small increase in regeneration. This regen boost will strengthen as the amount of Hallow grows. In hardmode, the amount of Purity in relation to H/C affects how long the buff will last. 0% purity is 2 seconds, 10% is 3, 20% is 4, and so on. This is doubled if there is no C in the world. If the world is completely pure, the buff will last 24 seconds, but there will be no regen (As there would be no Hallow). By controlling the amount of Hallow, you could balance the duration and relation to regen strength.
I don't really like the idea of the Ultimate Sacrifice. There really isn't a good way to balance that sort of thing. I don't like the idea of the Dryad only appearing at night or Solar Eclipses (When NPCs are most likely to die), either. Instead, the Dryad, when given the souls and luminite, should slow or stop biome spread, similar to how the spread slows when you kill Plantera. She should also remain alive. This would only work in a world where all bosses are defeated. Also, "Souls of Delight" are not necessary, because there's luminite. There's no need to add an extra drop that's only used for one purpose that can only be done once that another drop from the same creature is already fulfilling.
As for the Arms Dealer, he shouldn't require the Nurse to be in a house, because he was also hitting on the Dryad. The quest item shouldn't be a strange plant, life crystal, fireblossom, shortsword, or frog cage, and should change every day. The sword, fireblossom and crystal contrast greatly in difficulty, even though the crystal would make sense. Frog cage... don't know why the Nurse would want a frog. The strange plant is already a quest item, so it doesn't need to be used for another quest. I think the Arms Dealer should have multiple quests. The first would unlock reforging. The second, which WOULD require a heart crystal, would unlock Illegal Gun Parts (I was thinking about incorporating the Diamond ring in a different quest, but the Arms Dealer doesn't have anything that valuable to offer).
The Insanity Meter for the Clothier seems nice in concept, but it's actually pretty unnecessary. The reason the curse reappears when you use a voodoo doll is because the doll is imbued with cursed energies from the dungeon. Plus, the Insanity Meter would be difficult for a beginner to figure out. Even if the player does, what if they build a house at the entrance of the dungeon? The Cultists wouldn't spawn, and the player would have no clue what to do after the golem. Cultists are very picky.
Holding back easily-available triggers until you accomplish a tedious and pointless task makes no sense. Not only is the dungeon filled with pressure plates and switches, but the quest is completely unnecessary. Think about it: A player is asked to use 100-300 wire-related objects. What does the player do? Walk outside, place 100-300 wires, break them, and return. See how pointless that was? There's not too much use for wires until you get teleporters, unless you go out of your way to build farming structures (Usually, the most I build is a couple wires for a slime statue farm. I know the minion's not very strong, but I got it once in my first playthrough back in 1.2.1.2, it helped me beat skeletron, and I grew attached to it).
The Goblin Tinker shouldn't require 5 accessories to spawn. That would be fine if you didn't have to look for him (And I'm not implying that you shouldn't), but it doesn't seem right as it is. I don't think the reforging should take materials, because that can get too complicated and tedious. "Breaking" shouldn't be an option, and the Broken modifier should remain the same. What if the player starts with a broken tool? However, I do think the reforging should be less randomized. Perhaps the modifiers shouldn't be allowed to rise/drop so quickly? The Workshop buff is nice, but I don't think it should be there.
I love the Wizard's ability to sell ingredients, but I don't think he should sell stars. In hardmode, the amount of stars I usually pick up increases, and my use for them decreases. It takes me only a few in-game days to make a skybridge out of platforms, and the stars really pile up. I've gotten 99 just making the bridge.
The Steampunker should not appear until a Mech has been killed. In fact, she shouldn't sell teleporters until all 3 have been defeated. The solution location just seems unnecessary. I try my best to keep these NPCs together. I think the items required to make this "Clen2minator" should be Iron/Lead and Souls. The player should already have a supply of these materials, especially if they're farming mechs. Also, +5 range is a negligible buff. I think it should have a wider spread, making it less likely to miss some blocks.
While players with a Witch Doctor would most likely have a Dryad, requiring a Dryad for the fountains to work doesn't make much sense and hardly seems intuitive. Other than that, I'm fine with these changes.
The Pirate's fine, but does the invasion change remain increased, or does it return to normal after the quest?
"This can only be done every in-game days" I'm just going to assume you mean every day until I'm told otherwise. I having nothing to say about this suggestion, as it seems fine as it is.
I think the glowing truffle should spawn in any mushroom biome (surface or underground) once you have a surface mushroom house, just to make it a bit more challenging. Surface mushroom biomes are usually a bit small compared to their underground counterparts. Also, this suggestion seems to have a lot of waiting. The Truffle should sell his items as soon as he grows up. He shouldn't leave for a few days, either. The player should unlock items depending on how many glowing mushrooms he gives the Truffle, with the pet being unlocked last. The requirements for the items should stay however (Autohammer should still require Plantera's death, and so on).
Due to his short appearance, the Travelling Merchant's stock should be infinite, but he should have limited money. Also, he should buy anything. I think that's all I have to say about him.
Closing words: Well, I probably just wasted about 2 hours of my life, but it was worth it. Overall, this suggestion made a nice effort to improve the interaction between the player and NPCs. The grammar could be better (By the way, "vacate" means to leave
). Also, the banner suggests it focuses on interaction between NPCs, so that could be changed. And the Dye Trader shows up twice on it.
