After the other aliens kicked humanity out of the „space club“ and considering what they tend to do to their „discontents“ i also don‘t believe doomslug to be a watcher.
I like the idea of it being the former pilot though. With the option of switching one‘s mind with another as read in DE, even between different species, i think it may be possible that the pilot - which i believe to be cytonic - switched his mind to another species. Doomslug immitating the human language may be him trying to relearn what he has forgotten due to the mindswap.
I still need to re-read Skyward and as english is not my mother language i may very well have missed or misunderstood something.