1. There were no end of complaints about leaderboard ratings "abuse" so Lichess did the simple, obvious solution which I think is quite reasonable. There might be a more complex solution but it would be difficult to test. 2. Interesting although I can't think of a good UI way to make this work.
1. It's an understandable reason although it's a bit pity that the games with the longest time control and may include the largest amount of calculations by players doesn't count in points that shows the players' strength.
2. For now, when I find someone create 2 challenges with the same setting, then I join in both so I can have a white and a black game (although it's possible to have the same color). I mean it's nice if lichess server can automatically do it, not something like to show white game in the left and black game in the right of the same page. I think it's not that difficult to code although I'm not a programmer.
1. I think the main reason is that, sadly, correspondence is by far the easiest time control to cheat in without getting detected. No way to avoid that standard correspondence ratings are influenced by this, but otherwise correspondence is kept in isolation so that it doesn't "infect" variant ratings (which, unlike standard chess, mix time controls).
2. Could be as simple as a little popup when you enter a game that asks "Play another game against this opponent with reversed colors?" and if both agree, the game is started. Then again, that's not really less work than simply challenging them manually (you can set the colors too there).