Faster mist energy recovery means that you have to play the game more frequently to avoid "wasting" mist energy, which is a bad idea. There are other ways to get the same effect (e.g., double mist energy recovery speed plus a doubled mist tank capacity, or halving the cost of everything). But there's also the question of whether this will cost Three Rings revenue, and the answer is probably that it would. Which is why they probably won't do that, except for short-term promotional things that give them an excuse for some publicity to try to attract new players.

The difference in crowns between tiers isn't much, but you get a lot more crowns when you play an even stratum (past terminal, before town/core) in any tier. Jelly runs for example cost 30ce (if you only do the palace) and get you around 4000 crowns. Yeah, chances are you'll join a few depths before the even stratum, but nevertheless you can make more than enough to spend every waking moment playing the game if you so chose.

tl;dr end of T2 is probably where you want to 'farm'. Any gate works fine, but jelly palace is best.