If you add a downed pilot manually in the mission editor (I don't remember the how right now) then he cannot be picked up without the appropriate attached trigger (approach type) which has a "remove attached object" action (or runs a script that does it)
As for those that are auto-generated when planes are shot down, I simply don't know. I cannot remember being able to pick them up since fleet command, and there things were a bit different.
As for the DSRV, no. It's only supposed to attach to other subs, though that isn't exactly done correctly either.
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Teaching DW newbies how to climb the food chain.
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