Oh, I'm not saying those dossiers are definitely canon. I take them as such because they seem legit, but it's really anyone's call, not that they have much bearing on anything unless you want them to.
I think Dallas just seems worn down. He doesn't seem like a weak man, per se; he's just been on the job too long and, as he tells Ripley, wants to get the hell out of there, and not just off the planet. He also appeared to have a longstanding professional relationship with Kane - or maybe I'm misremembering some point from the novelization years ago, that may be bullshit - and broke quarantine and was lax on the rules to let him back in. Janek, we can't really say what kind of character he is beyond a few inferences from the actor and what we've seen - focused on the job, prestigious assignment, and lighthearted enough to install a Christmas tree onboard.
