Can it hurt a 5.1amp only using left & right chnls

No problem.

Nope, just turn off the unused channel in it's menus, if possible. Will be something like 'no' in the places where it lists the crossover slopes Won't damage anything either way.

No. Call mfg and find out if you can bridge the channels for more power. May not be possible on your mid fi reciever

Yes, but of course those unused channels have "hurt" feelings cause they've been neglected. Where's the love for those wasted rears?

All kiddin' aside, the HK I owned for a bit ran warm too. Perhaps it's the nature of the amps inside, but as long as you're providing it ample ventilation I'd not worry about it.

Enjoy!