Too useful against bosses. 2 (or 3) uses would kill everything.
Damaged would have been based on current HP, so I doubt spamming use would have been very useful.
Anyway, bosses would still need an exception, and I'm not sure the item would be that great.
For skulls, I like the portable repair idea, but I'd rather couple it with blood skull (healing your gear, somehow :p)
My suggestion for fire skull would be to
- register every explosion before the first occurs (so that one exploding corpse doesn't prevent 4 other from exploding. ALL skulls in range would inflict damages)
- change damage type to "holy fire". Something which wouldn't damage player or destroy items. Maybe not walls either.
- Improve damages if necessary. The idea is to "nuke" the screen when it's filled with corpses. Outside of the mortuary, such situations just don't exist, so it would still need to compete with hatred skull. I'm not asking 100 damages per explosion, but a vile between 4 or 5 corpses should need improbable damages rolls to survive.