Have most atheists honestly put thought into what if there actually was a God?

Don't know. Most probably have something better to do. I have thought about what would happen if there was a God. If it turned out the the god of the religion I was brought up in was real then I would be destined to burn in hell for eternity. If version 1 of the same god (Yahweh) existed I'd probably also burn in hell for eternity but I'm a bit less certain about that because the first half of my Bible talked more about punishing people while alive (well, at the start of the stoning they are alive at least) than the threat of torment after death. If Alah is real... well, I'm guessing there is going to be more eternal pain involved since that is just another fork of the same counterfactual omnipotent psychopath. Maybe I'd have more luck with the religions from ancient India---so long as I can convince the gods that lesswrong Karma counts.

So yes, I've given some thought to what happens if God exists: I'd be screwed and God would still be a total dick of no moral worth.

Many won't even accept that there is a possibility, and I think this is just as dangerous as blind faith.

Assigning probability 0 or 1 to a hypothesis is an error, but rounding off 0.0001 to 0 is less likely to be systematically destructive to an entire epistemological framework than rounding 0.0001 off to 1.