This also would require that devs who are on the PR reviews ACTUALLY look at the code. In about every job I've worked at in my short career there are people I work with that I don't trust them to actually review my code. I've come to accept that. I instead make sure the people I know will do a decent job are on the PR. Some people will just look at the diff and an even smaller few will actually pull the branch locally so they can see the entire context. That being said I always do my best to review other peoples code regardless of whether or not they will review mine.

Not really. It just ensures no really atrocious code makes it into master.

Of course you can stall any junior merge request until it looks like senior code, but at that point you might as well write it yourself.

I don't know if it's stalling so much as taking the time to request changes / pair up and teach.

If bad code is getting merged, that's tech debt / time someone else is going to have to spend anyway, plus the time needed to identify the issue and triage down the line. I would think it's better investment to use that time up front and help the jumior level up too.