I went down a thought exercise based on flawed assumptions in the OP’s statements. SR and MMR do not drift apart except in the cases of DC penalties and Decay.
Alts and smurfs are different. Alts just haven’t gotten to where they belong yet. Smurfs have had their SR/MMR intentionally manipulated, and damage everything about the matchmaker.
Fortunately, in Silver+ the Alt/Smurf problem is small enough to only impact people occasionally, and it’s random whether it’s a positive or negative impact.
Smurfs unfortunately have a very outsized impact on low bronze (500-800SR) where the thrower trolls go to destroy games endlessly. The population is so small there that they have a significant impact.
Repeating: SR/MMR do not drift apart even for throwers. The matchmaker does not do anything but attempt to match 12 people with the same SR/MMR into a game.