Hiya Redroach.
Yes one 250lb bomb is enough. I try to aim for the part of the hull where the funnel is. Seems reasonable to aim for the engine room!
As to bomb drop, it's been a long time since I set up my controls, so I can't say if it's default or not, but I have a button set to drop individually.
One thing I have noticed though is that the loadout can sometimes default to 2 x 500lb if you don't check before take-off, which sometimes leads to confusion.
Meantime, with the new patch, the flying aspects of these vids are out of date, but I'm not going to change them until I know that the FM is finalised.
Cheers!