I thought this back in the novel, and I still think this here: The whole "make mc a king, then suddenly avoid him like the plague, while conspiring to make his best ally a pariah to the state for some cheap shot at purging nobles I already know the name of" thing feels utterly stupid. With all that future knowledge he could have easily handled things differently enough to handle things adequately, and even if he makes mc king for his own conscience (what? why? mc clearly doesn't want to be king and he should have known that) he could have offered his utmost support from the get-go to have things end much differently and better. For starters, telling mc about everything from the start would have made a lot of sense.
Let's say he instantly decided to purge all of the traitorous nobles.
First, how ? Would he invite them all to the capital and have a Hongmen-style banquet (ie killing everyone ) ?
Tyrant, 3 dukes would try to depose him or the noble heirs would revolt or Amidonia would come in or all and more at the same time.
Send the troops to defeat all of them ?
Tyrant, but also starting a civil war in dire situation already and Amidonia would invade and maybe more.
Do them one by one ?
Tyrant, still need reasons to do it and they might get wise / heirs might pick up the habit, and he still wouldn't have that much time.
Can you think of one way to legitimately , stably and effectively remove as many nobles as this involved (never an exact number, but the fact that all but 2 of the council of high nobles or whatever he got for the trial jury were the fencesitters, which meant atleast partially neutral... ) without anything other than Treason, Rebellion or Glorious death in combat (that one's impossible, none of them seemed to be able or want to fight in the rebellion and even then all of them are impossible)
Crushing actual rebellions is the single most effective , legal & stable way to really get rid of various people if the evidence is there.
Also, you're forgetting that he was already convinced by them last time. Sure, the transfer memory would affect him, but he might still be convinced to go easy on them if he tried something and they could just start a new civil war at one time or another.
And if he had told the MC about the traitors, the MC might've actually made it the single focus from the start which would've doomed the whole restoration, not to mention whether he would've believed it like 'We just summoned you here, our past memories of this say these people are traitors and need to be purged, you'll love and treasure our daughter and avoid war', that would really freak someone out.