in this case, an old helicoil repair now casuing YOU trouble, i would cut the nut off, remove the manifold which will you much more room to see whats happening to resolve the problem.

a drill with bits 1/16 thru 3/16" will gut the nut or a cold chisel rapping it a few times.

you can also remove all the other manifold nuts save this sticky one. then use a pry bar with the business edge rapped in tape so as not to scratch the manifold or head and insert it between the manifold and head . using leverage apply outward force against manifold so as to make it tight against the offending nut while loosening the nut simultaneously.