Some of them are unproved, none of them are impossible.
> 1/ the existence of life outside earth,
Life spontaneously emerging on earth implies at the very least the possibility that it can happen elsewhere
2/ the existence of technology outside earth,
Same as above, Crows and Apes using tools implies technology is a possible result of life.
3/ the possibility of FTL travel,
Nope, those probes could very well be subluminal, see for example: https://en.wikipedia.org/wiki/Self-replicating_spacecraft
4/ the regulat visit of alien on earth,
Not regular, we just have to notice them at least once
5/ having a technology good enough to be almost undetected but not completely
Which is literally what every stealth tech we have made is.
6/ having an irresistible urge to crash in the US
There's probably 10x more UFO reports coming from outside the US, especially south america, than from inside the US and I have made no claim that I have heard any good evidence that any crashed UFO has been retrieved (though we have crashed some mars rovers in our time so it's not out of the realm of possibility that an alien species might crash their stuff on our planet)
We do have the proof that North Korea and nuclear weapons exists. The US government could be lying about the tests like it lied about iraqis WMD but 1/ there are other sources 2/ its in the domain of plausability anyway.
At the reverse, alien origin of UFO relies on unproved and impossible facts: 1/ the existence of life outside earth, 2/ the existence of technology outside earth, 3/ the possibility of FTL travel, 4/ the regulat visit of alien on earth, 5/ having a technology good enough to be almost undetected but not completely 6/ having an irresistible urge to crash in the US