According to this video the merchant does not care about the other npcs, only considers the biome and the nurse in my case:
But at that point maybe things shouldn't be the same then
It's too hellish to me, even thinking about going back to my previous game, leaving terraria to later
That was a pretty interesting video, and I get now, that it talks about "villagers" who are nearby, but not "neighbors"
But, I think, the info/description on youtube says, that these apply to 1.4.0.3, and the happiness system was changed like twice since that, so I'm not sure what are what applies, and what are that doesn't.
I think, NPCs don't mind having a few more neighbors now. but also think, that it's not possible to get some of the pylons in the ways that worked in 1.4.0.3. for example, you can't get the stylist to sell the ocean pylon just because she's in the ocean and has plenty of space, she needs an NPC she likes, like the dye trader at least until you buy the pylon.
I wonder if the biome + space + 1 liked NPC would make the merchant sell the pylon... but I think he won't sell it. or I'm not sure. (yeah, there is also that, some NPCs are "liked", some are "loved", so loved ones add more hapiness bonus.
I got merchant to sell the jungle pylon just earlier with space + nurse + golfer, and then moved them elsewhere to buy the mushroom pylon too. and it probably works with several other biomes too, which makes the whole thing kinda defeat the whole purpose of caring about the other NPCs happiness enough to sell the pylons...? (but not the discount in prices.) (the wiki states he dislikes the desert, so it might won't work for the desert, but the arms dealer + nurse, or dye trader + arms dealer combo should get the desert pylon the easiest and quickest, usually.
and if the merchant + nurse + golfer trio works for every other biomes, it can be a very fast, and kinda "cheap" way to set up all towns very early game, (except for hallow, since it's not normally present at the start of the game)
didn't tested it but I think it could work, huh.
That's a bit off topic though... but I wonder... hmm... according to the wiki, the nurse and the golfer are both "liked" NPCs, as he has no NPCs he "loves". So, they likely don't give enough bonus, making the most likely strategy to work - if centered around the merchant - that you need the space bonus, the nurse and the golfer (and then the biome doesn't even matters, at least as long as it's not the desert)