Can I use Demise's effect more than once per turn?
Or do there need to be cards on the field (other than him) in order for me to activate his effect.

I ask this question becuase I want to drop my lifepoints from 8000 to 2000, so I can then metamorphasis him into Cybertwin, then megamorph cybertwin to become 5600. And since Im at 2000, he stays at 5600 instead of dropping to 1400.

Looked around first but couldnt find the answer.

As long as there is at least 1 OTHER card on the field, you can use it as many times as you please, but if you blow the whole field up the first time, and want to activate it again without any other cards you just ran out of luck.

Limiter removal would work, as your lifepoints wouldn't matter. Or, assuming you haven't summoned yet, after an attack at 5600 and another at 1400 (from the cybertwin) you'd only need another monster with 1000 atk to finish him off.

Play a card facedown to the backrow, then pay again. problem solved.

^ exactly what you should do^