Welcome to A&A.org, MNGruber
Couple things
Units hit by shore bombardment DO get to return fire. They are not like submarine surprise strike victims.
Subs only roll once per round as Boldfresh pointed out. If they have a surprise strike capability, then they roll first, but not again later.
Generally, subs submerge when they would normally be rolling dice.
If there is no enemy destroyer present, the sub can submerge before any shots are fired by anyone.
This means that if you do not have a destroyer present, you can NOT EVER force a defending sub to fight. It can always immediately submerge. This includes if you have subs yourself (but no destroyer). You can’t make a defending sub fight without having a destroyer yourself. As soon as you lose your last destroyer, the next round, the defending subs can submerge. (Same would be true for attacking subs)
After the round that the last destroyer is sunk, any remaining enemy submarines (whether attacking or defending, it does not matter) will get their special abilities back. (Submerge, surprise strike)
If this doesn’t completely answer your questions, feel free to ask follow up questions and someone will help you with them.