I have a dumb question. Is it at all possible to get an LE-rarity mech (and i dont mean the grey-based ones, just the black based variants) in a booster?

I assume you mean from sets before Falcon's Prey? Is it possible? Yes, anything is possible. If it happens, it's a factory error and was not meant to happen that way. It doesn't make that piece special, it just means someone at the factory goofed up.

Is it possible to get an LE figure in a falcon's prey or liao incursion booster (figures listed as LE 5 or 6 on the wizkids techincal readouts on the website).

Falcon's Prey: yes

Liao Incursion: no (other than an error)

Thanks for clearing that up.