@Tizkit
You can split up units as usual. If any attack ships stay in that sea zone it becomes a combat zone with you as the attacker.
The main issue is ships must move on the combat move phase rather than wait for non combat and if the zone becomes a combat zone, you couldn’t pass through it while making moves on the combat turn.
Example, if this were in the English channel and you wanted a transport by Gibraltor to attack Norway, you would have to avoid making sz110 a combat zone.
As for your empty transport, it cant load in that sea zone, but it can leave, pick up troops, and continue movement as part of a combat or non-combat move.