I'm new at SH4 too
Haven't been in real combat yet, just the training missions.
Regarding your 2nd question, some ships have a VERY strong will to live and take an ungodly amount of time to sink. Others give up easily and sink in a matter of minutes.
Well actually, I imagine it's more to do with where you hit 'em. When playing SH3, I hit a C3 freighter once and stopped her dead in the water, she was still floating 24 hrs later. Not wanting to waste torps (was nearly out) I finally surfaced and took her out with the deck gun.
About your crew dying, did you take them off battle stations after the engagement was over? Think I read somewhere yesterday or today that if you leave them on battle stations too long, they'll die, basically drop dead of fatigue. They're
very dedicated crew apparently
Maybe that also has an effect on how well they do repairs? If they're dropping from exhaustion, don't know how well they could fix things.