I feel like I can say with near absolute certainty that the delegates who actually vote would not allow this to happen

The electors are chosen by the party at the same convention where the candidate is nominated. If the party opposed their own candidate they would just not have nominated them in the first place.

and even if they did, another entity with more power would stall the process and/or change the rules, and everyone would accept that as normal.

You’re right here though. It’s up to each state’s government (usually the state Attorney General) to finalize the result of their election and thus pick which slate of electors they send.

Alternatively, it’s up to Congress to actually count the electoral votes and finalize the election. That’s what January 6th was about. They have since passed a resolution saying that part of the process is a formality that can’t have an effect on the result, but they’d absolutely ignore that if necessary.

But it depends on who that “left populist” is. If Bernie had won, they likely wouldn’t have gone that extreme but rather just stalled his policies in Congress and the courts. He’d get some basic (and acceptable to capital) progressive policies through via executive order - like descheduling weed - but he’s too much of a lib to do anything much further left than the New Deal. Remember, Bernie’s policies wouldn’t stop capitalism but rather save it from itself (temporarily ofc).

If Claudia de la Cruz somehow won, yeah they’d probably stop that.