If one is incapable of doing evil in heaven does he have free will?

What evil could you do in an eternal paradise? And similarly, in a paradise, what sort of temptation to evil do you foresee?

what makes heaven different and why it wouldn’t be possible right here right now?

Again, if you want to reward people who are good, you need a proving ground. In a paradise, anyone can be good. When life is harder (or has more temptations) that’s when our character shines through.