" he has the power to lie, but his nature restricts him from doing so "
When I was a JW, this is pretty much what I had in mind. I guess the question that would pose would possibly be: "Could be he capable of going against his own nature, even if he wanted to?"
I don't personally think the bible does point towards an all powerful God, though many JWs who haven't taken the time to think about it would say it does - I don't think JWs like the idea of a limited God.
The same goes for omniscience - if that is the case then everything is pre-destined. I think both omnipotence and omniscience are ideas borrowed from elsewhere, most probably Greeks, but I'm not sure.
Saying that, I'm not even sure myself at the moment in the existance of a God at all even, but that's a separate thing.