If the fellaxe is disposable we can just send a goblin on a suicide attack. The other three will just have to watch the axe go in, jot down some notes, and then go home.
Yes, the artifact from our deity is disposable. Just like people don't hoard alleged bits of the Cross IRL, if those bits also had great practical and magical value.
When the lifers come visiting later we can just say some idiot of a goblin stole our treasured artifact (when they eventually make the Fellhawk - Fellaxe connection) and thank them for killing him.
I'm not sure they'd be that dense. Or that they wouldn't just destroy the Fellaxe.
If goblins come out of the filth birthing pit, they're disposable. Losing one for a greater bad won't hurt.
Probably true, but we don't know how the Pits work. If we have to refill the pit after each birth, it would at the least be
annoying to keep making new goblins.