I was in USAF a few years after Belenko defected. At that time he was employed by USAF as a speaker who presented a class on Soviet thinking to fighter pilots attending Red Flag. So I met the guy, albeit briefly. As did dozens of other people that morning.
USAF / DoD did have quite a process at the beginning to decide how much to trust this guy and his story. The MiG-25 was unequivocal evidence, while everything he said might have been BS or a Soviet ploy to embed a double-agent in USAF.
IIRC he was still utterly untrusted when I came by a decade later. Treated with respect, but at the same time as a “useful idiot”. But he had a comfortable living and a US lifetime pension, so he was happy enough it seemed. I later read his book which pretty well corroborates the mindset I saw on display that one afternoon. Both his and that of USAF.
As to the OP’s overall scenario, we’ve done this a time or twelve.
The First Contact, when, not if, it becomes public knowledge will rock our World even if an alien fleet never shows up to issue an ultimatum. The collective insanity engulfing the US today is nothing compared to what will happen when alien(s) are acknowledged to be here.
As to the ultimatum, we have no rational choice but to comply, while at the same time expecting to be incinerated, conquered, or interned / quarantined anyhow despite our compliance with all demands.
This is the practical equivalent of a SWAT team showing up at a little old lady’s house to arrest her. The least bad possible outcome for her is they get exactly what they want without too much collateral damage to her.
However an alien fleet up there making threatening noises will make humanity totally lose its collective shit.
As @puzzlegal so wisely points out just above, who is “we”? The Earth is not going to speak with one voice in this. Even if the human public wanted to coalesce around a single coherent decision (which they won’t), it would take far longer than the aliens are likely to wait for the Earth governments to get their collective shit together.