Awesome. Seems thoughtfully made.
So if you don’t play for a while does your MMR decay like your LP does?
That's a variable we tune separately from the ranks. Sometimes it decays, sometimes it doesn't, but we base that more on whether or not players are actually worse than expected after not playing for some period of time.
I think it's currently possible in some cases for your MMR to also decay resulting in worse LP gains on return, but I can't guarantee that it will always be the case.