I have never worked with PVC, but styrene it all depends on who makes it, how well it is molded, and how thick it is...it does have a shelf life, but made properly can last a long time
no, I don't think so it would be very uncomfortable, and heavy...it would really put a downer on your first troop if you can only last 45 minutes in your helmet
I wouldn't look at it as having problems with trimming, I would look at it as having allot of fun customising your armor to yourself, so it is no one elses, but your own creation