GM has heated cloth and leatherette seats on many of its trucks. I think the decision to package heating and leather together is for marketing reasons, not because of any technology-compatibility issues. (They're both "premium" options and bundling them together makes sense if you are constructing a multi-thousand-dollar package.)

Leather seats have the same padding below them as non-leather. It isn't a solid chunk of leather, just a leather outer layer.

So in answer to your question the fire hazard of both is identical and many MANY vehicles with non-leather seats have seat warmers (including mine).

Not the person to whom you're replying, but I've looked at multiple cars with factory heated, non-leather seats, so I would assume so.

Of course. Both of my cars have cloth heats with seat heaters.