Glad to hear you're enjoying the game! Sorry I made some of those achievements quite brutal.
1.) Mage Hand lets you interact with blocks at a distance, and you set it up with runes to target a block the same way you would set up a teleporter. You can use it to interact with a chest at a distance, for example. I'm not sure it's that useful but I figured it might be used if someone got around to making a custom map, since it would let you hide exactly what the player is interacting with.
2.) Anything that drops a painting can drop any painting, they're not set to any specific map or boss. If you're going the fishing method I recommend getting an eyepatch from lava fishing first, otherwise just farm the boss that takes you the least amount of time to kill.
3.) Nope, just aesthetics! Tabi redid my art for bricks/obsidian bricks and while I liked it it felt more metallic than stone, so I added them as iron/dusk brick variants for decoration.