"A flat change like this is fine if we are resetting what a certain elo "means", which seems to be what you are suggesting (bringing the ffa elo in line with 2pc)."
You got it.
"how well can 2pc & 4pc ratings be compared"
that's the limitation of this all. I personally think they are close enough there is value to this idea. It won't be perfect; I think just getting it generally correct is acceptable.
I agree with your comments of statistical analysis and inflation
"adjusting for TC + playerbase"
The playerbase reacting negatively could be a problem, yes. I mean it shouldn't be, but it could be. As for adjusting for different time controls, I am not sure; that is a valid issue. I was thinking just leave blitz, bullet, and hyper alone for right now, until they have a larger and less volatile player base. I don't see much harm in having the rapid ratings lower and keeping theirs the same. Alternatively, they could be raised or lowered by smaller amounts for right now.
Thanks for your thoughts!
10.05.2019
there was hardcore time, I couldn't even reach 1900
although we already used cooperation in its infancy https://www.chess.com/variants/custom/game/1152492/56/1 (-300 to all ratings)