Verify authenticity of the Star Wars Snowtrooper minifig
It's hard to be certain without some more pictures or information to narrow down when these minifigs were produced, but it sounds like what you have are some of the older Snowtroopers with the helmet and backpack attached in a single piece. Based on that assumption and the details you provided, I think you are looking at authentic Lego, with the possible exception of the blasters.

Point by point:


I couldn't find a LEGO logo in any of my helmets, but it did look like there was a copyright icon in the central anti-stud.
Many of the older snow/storm/clone troopers have blank, black heads. The ones that do not have a solid central stud hole (but a triangular arrangement of holes) do not have a LEGO logo.
Your description matches my torsos. All have the LEGO logo on the top of the neck stud, somewhat obscured by what I assume is where the mold injected the plastic.
The legs all sound legitimate to me. I was able to find minifigs that matched the variations you describe.
These seem suspect. I was unable to find any in my collection that matched these patterns of printing. In all of mine, there was printing in the recessed "bolt" section of the barrel. None of them had printing on the trigger section.