1) and 3) (and, to an extent, 2) )are routinely done, to some degree, by your average security-conscious employer. Your employer knows who you are and probably put some thought on how to avoid your accounts getting hacked.
But what is reliability? Could be anything from "this dude has no outstanding warrants" to "this dude has been extensively investigated by a law enforcement agency with enough resources to dig into their life, finances, friends and family, habits, and so on".
I might be willing to go through these hoops for an actual, "real world" job, but submitting myself to months of investigation just to be able to commit into a Github repository seems excessive.
Also, people change, and you should be able to keep track of everyone all the time, in case someone gets blackmailed or otherwise persuaded to do bad things. And what happens if you find out someone is a double agent? Rolling back years of commits can be incredibly hard.
If the ultimate goal is to avoid backdoors in critical infrastructures (think government systems, financial sector, transportation,...) you could force those organizations to use forks managed by an entity like CISA, NIST or whatever.
If the ultimate goal is to avoid backdoors in random systems (i.e. for "opportunistic attacks"), you have to keep in mind random people and non-critical companies can and will install unknown OSS projects as well as unknown proprietary stuff, known but unmaintained proprietary stuff (think Windows XP), self-maintained code, and so on. Enforcing TS clearances on OSS projects would not significantly mitigate that risk, IMHO.
Not to mention that, as we now know, allies spy and backdoor allies (or at least they try)... so an international alliance doesn't mean intelligence agencies won't try to backdoor systems owned by other countries, even if they are "allies".
