I think this scripture is not nearly as clear in its meaning as you seem to think.
If disbelief is the sin Paul refers to here, this presumably means we should NOT pray for those who are unbelievers.
Which pretty much contradicts every sermon I've ever listened to where the point is made that we should pray to God to open the eyes of unbelievers and bring them to belief.
I also think disbelief, in the sense you use it, means more than absence of belief. I think it means believing and rejecting. The Bible says the demons believe and shudder. That's quite different from a lack of belief as such.
Perhaps we should call the absence of belief nonbelief and the belief/rejection anti-belief.
Prayer to soften a bear or open eyes to the gospel, yes. How would God grant forgiveness for disbelief in himself ? The individual has chosen separation from God, which is death.