Old plumbers trick . . . when trying to remove something that is threaded on TIGHT . . . first turn it RIGHT (to tighten) This breaks the threadbond and THEN try to back it off. Ive used this on everything from pipes to engines and it works a good bit of the time.
Also when I installed that stock I removed the black plastic piece insid the button spring and cut1/8 " off and replaced. This allowed enough clearanceto press the stock adjust button enough t slide over the end