Salsa,
You can use your MTB with the Spinervals but you may find gearing will slightly be different (mine is already different as I've got 650 wheels so I adjust accordingly). The only thing about the MTB on your trainer (if you've not done it yet) -- is the tire is loud so suggest perhaps a putting on a slick tire while you are indoors.

Changing hand positions is Coach Troy's way of breaking things up a little and he even says "if you're a triathlete, go into onto the aerobar, if you're a bike racer, go into the drops" etc. So he's just trying to get you used to hand positions you'll need for your sport.

I really like his newer DVDs (Aerobase builders etc.) that allow you to turn his music on or off, so you still hear him, but not that cheesy (sorry Troy!! ) music. Turn on your stereo, pod, whatever, it keeps you engaged. I can only do those looooong rides (2 hr) if I am doing a recovery ride, and then even watching a movie while maintaining a zone 1 heart rate works well.

Troy also puts his workouts of the week every week on his site (they don't archive them tho so you need to grab them every week), so if you want to try one of his workouts, you can print it and put it to your own music. A good way to try his routines.

Have fun til the warm weather returns!