It's always been a worry of mine that i'll practice going sheriff only, but that I'll do badly and get paired up against worse people and get fake confidence from doing good all of a sudden.
Everything uses MMR, but some modes are more loose than others - so whilst yes, it does, it crosses over with MMR from other modes. I've done sheriff only DMs and there hasn't been a major difference in ranks besides when I've actually ranked up in competitive
u do have mmr in dms, i've been in full immo dms and full gold/plat dms at differing times, there's defo mmr it just depends how well u play tbh, it's also very loose though, win 1 dm insanely dominantly in like gold, expect to be in high dia next game