Welcome, Granada!
@Granada:
1. Can a submerged sub attack move than conduct opening fire on enemy ships in the particular sea zone without them having the possibility to fire during the defending units fire step (providing there are no destroyers or submerged subs), since the sub is submerged and hence “not visible” for them?
Submerged subs can’t attack or move. They remain in the sea zone “invisible” until they surface again. Submerging is a deep dive and is a defensive mechanism.
@Granada:
2. What about a surfaced sub attacked by another sub? A. Has a surfaced sub the possibility to fire back at all, or B. does it also only fire in the step 2 (conduct opening fire), or C. in the step 5 (defending units fire)?
Subs always fire in step 2, whether attacking or defending. What differs is when the casualties from sub fire are removed. If there is no enemy destroyer, the casualties are removed after step 2. If there is an enemy destroyer, the casualties hang around until after all units have fired. In either case, a defending sub will still return fire if hit by an attacking sub, since both fire in step 2.
@Granada:
3. Is this repeated in the following cycles of combat? If so, does it mean that subs can sink any ships without the cover of destroyers or subs (providing what is the answer on previous questions)?
All cycles of sea combat work the same way. If one side has subs and the other has no destroyers, the casualties from sub fire will be removed before they can fire back, but ships that aren’t hit will fire back.
@Granada:
4. Can a sub submerge without being attacked?
No. Submerging happens at the end of a combat round, so a sub must endure at least one round of fire before submerging.