happened to me before.
i greased my threads, installed my tube, properly torqued castle nut the right way, 90* 40ftlbs, i couldnt remove it when i tried to, not staked. used 3 different wrenches and they all slipped. held square etc. im very mechanically inclined.
name brand nut too.
couldnt get it off.
so, i used an 18" pipe wrench, the nut cracked in half and fell off as i loosened it. no damage to receiver. some the threads were gouged on the extension tube.
i wouldnt worry about the threads on the extension tube its fine. but do you personally want gauged up parts on your gun if you dont have to?
i always clamp the tube in the vise when torquing the nut down, i dont like the flex and stress on the receiver using a magwell block.