I'd be fine with an AC change but a full on class change. No way. You chose your class there's no going back. If you want to play something else then re roll.
Actually Powertechs are one of the least played classes
http://dulfy.net/2012/03/15/swtor-overall-advanced-class-distribution/
not sure why they wouldn't be recruited though.
I assume you are talking about the black hole stuff. Which isn't the best in the game, doesn't have a set bonus, and takes about a week to earn a single piece.
I'm actually really looking forward to taking them on. I feel like they could be excellent villains.I won't feel guilty fighting like I did with malgus.
Can't say for sure but I have never run into this. There are a few areas you return to on Dromund Kaas that are higher levels but they are all connected to your class story.
Not that I know of but the level can serve as an indicator of planet. If its 33=taris/balmorra. If it's 45= voss. You get the idea. It's not perfect but it helps.