"See that fine line that's about 3/8" behind the bolts lug?
That line marks where the locking lug section is brazed onto the bolt body on those bolts!
Will they fire if the bolt handle is closed but the locking lug section's braze has let go?"
Not sure.
Whether it did or not is irrelevant at least regarding the gun going off. The locking lugs would do their job. Whee it might be a problen is if the shooter was facing someting like an enraged mama bear that thought her cubs were in danger. You'd best make that one shot count.
Seriously though, the only Remington problems I ever saw while working for a gunsmith was bolt handles that came off Remington 788s. I've heard of M700s sheding bolt handles but never the bolt head. Even if it sheared off completely odds are the locking lugs would be still in place in their slots so that nothing would/should blow back. I would also think that if such a thing would occur, it would be due to a bolt jammed in place by very high pressure and if that were the case the bolt handle would come off first.
Not saying that's a perfect answer BTW, just how I happen to see it. I don't think I'd really be that far of from what would happen. Even if the bolt turned to partially unlock. I do believe the lugs would hold as long as at least 50 per were still in contact.
As long as we're getting picky, I just checked out two Winchester M70s from 1968/69, a 1980's .300 Win. Mag. and two Featherweights, all push feed Winchester and guess what. The too had "welded bolt heads. (Actually brazed via an electrical induction process. Keeps th head and lugs from getting too hot while effecting a good hold.) If we're gonna pick on one, might as well pick on all the usual subjets, right?
Paul B.