When can you drive on it?

You can drive on it as soon as it is cooled, which should only take a few minutes. If you want to drive on it sooner, you could sprinkle a little sand on it.

Do cracks need to be fully dry before applying?

Probably yes. Maybe if you use a torch, use it to dry the crack. The stuff has to melt and fill the crack, otherwise in just lays over the crack. Just from my experience, I did it last year and it looked good, but this year the cracks came back. Maybe smaller than originally, so sealer maybe be enough to fill them this time.

Can you use a heat gun instead of a torch?

I don’t think a heat gun will get the sealer and pavement hot enough. I have been using a propane torch and it has been working quite well.

What temperature does it melt ?

Not sure of the temperature- burning hot. You need to use a propane flame/torch so it melts into the crack.. Something like Hot Max 500G Big Max 500,000 BTU Propane Torch