Got an answer from Capsicum Games:
Capsicum Games wrote:
a) How do I exit? I'm guessing exit per normal (space landed depends on the die). Or does an enemy ship on an exit portal block me?
Space Landed depends on the die. Ships directly on the portal square are not blocking the portal. To block a portal you need either to: occupy the entry, or occupy all the squares around the exit.
b) Can I attack the enemy ships through the portal? Or must I first exit the portal and then attack?
You can not attack through a portal. Even Special Weapons do not works from one Combat Zone to an Other (except Vortex Gate). You first need to go through.
So you can go through a portal even if there is an enemy ship on the exit portal. I suppose if you get a dieroll that lets you land ON the portal space, that would be the only time you'd be blocked.