1. Yes, I can see how people would feel that way, and if you want to stick with the current system you've implemented that's fine. I personally liked the old way but the new way is probably fine. I only ever saw the mini meteorites as a decent way to get more Luminite (a bit annoying to get..), as I have zero issues getting the other ores fairly easily. By that time I mostly just sell all the other bars since I have way too many.. Like I said I'm going to do a playthrough with the current new system and let you know how it feels.
2. I was thinking about adding some kind of chest. Maybe it would be a good idea to make them "themed"?
Corruption = Ebonwood Chest (can make in crimson world due to Living Loom wood exchange)
Crimson = Shadewood Chest (see above)
Frozen = Ice Chest (add a recipe if one doesn't exist, maybe 8 Ice Blocks + 2 Iron Bars @ Ice Machine or Anvil?)
Jungle = Rich Mahogany Chest
Hallowed = Pearlwood Chest
Another idea might be to create recipes for the actual biome chests, and use those instead of the above in the recipes. Something like..
Crimson Chest = Shadewood Chest + 10 Crimtane Bars
Corruption Chest = Ebonwood Chest + 10 Demonite Bars
Frozen Chest = Ice Chest + 4 Mythril/Orichalcum Bars
Jungle = Rich Mahogany Chest + 2 Chlorophyte Bars
Hallowed = Pearlwood Chest + 3 Hallowed Bars
The number of bars is meant to reflect the difficulty of getting each type of bar.
This would also kill 2 birds with 1 stone because currently you are only able to get 1 of each of those chests in any single world, so this would give players a way to craft those chests in case they want to use them for decoration purposes.
3. Yeah, I don't blame you for missing the Water Bolt, there are SOOO many RNG items in this game.. if I run into anything else I will let you know!
4. Absolutely, go ahead!
EDIT: As for biome key exchange, if you could come up with an elegant way to handle that, I think that would be a great idea! It could even be a crafting recipe..
1 biome key turns into 1 "Key of Purity".
Then you could do..
1 Key of Purity + 5 Crimtane Bars = 1 Crimson Key
Etc..
Making more uses for crafting materials that are completely out of date once you hit hard mode = great idea IMO!
EDIT2: Just so you know, if you can fix the meteorites dropping underground I think the problem people were complaining about (hoarding meteorites until post Moon Lord to get tons of Luminite) would largely be gone. That combined with the nerfed drop rate in hard mode would mean unless you specifically farm for the meteorites, you're going to end up with 20 or less by the time you kill Moon Lord.