I read an analysis of scenarios for the sub, and the best case is that they had a power failure but managed to surface. This is plausible because if something went wrong they would just need to drop their weights and float up naturally.

I'm not sure dropping the weights would help from that depth. There's a point at which the pressure of the water above you cancels out any buoyancy lifting you up. It's not even that super deep for divers; I don't know what it is for a duct-tape-and-zip-tied backyard-crafted submarine, though.