how can the buffer move past the back of the bolt?? Impossible unless something is really FKD up.. something else is your problem.
use vice grips and a piece of leather wrapped around the castle nut if you really want to take it apart.. but I cant see this being the main issue.
Do the take-down pins pull fully outward?? If so, nothing should be stopping the upper from coming off the lower except a little gentle bumping with your fist... and maybe a pulling forward as you pull up if it is the BOLT stuck rearward a little.
Why would you think it is the buffer?
If anything, it would be the BOLT that is not fully forward, causing the gun to lock shut. I would put money on this. Bang on the forward assist and try again