Thanks to Shiolle from the Paradox forums.
Nice thread. I'll try to contribute too.
Even though no one asked yet, I'll try to share what I learned about defense manager since it was not really easy for me to figure out. The name is a bit misleading since it's useful both on attack and defense. And it's use was not apparent for me when I first tried to wage war.
In SotS2 the space around a star and usually well beyond the orbit of the last planet is divided into small sectors. Every sector may be neutral (uncolored) or colored after any faction that has ships in this system. By default all the sectors belong to the controlling faction of the system and neutral (uncolored) for unclaimed system.
A player can place ships of his into any sector that belong to him (i.e. dyed with his color) during strategic turn via defense manager.
As to precise mechanics of the claiming process I'm not yet sure since I only fought in neutral systems and have yet to see how it's done when assaulting other faction's planet. The basic idea is that you claim any sector your ships move into. It seems any given sector can only change its owner once during one combat round. The details though are not that simple. In a neutral system when you move your ship into an unclaimed sector you indeed claim it as your own, but if you move into another faction space, you're making it neutral again, i.e. denying them territory. I'm yet to see how this works in a claimed system.
When a player first enters a solar system, it appears on its outskirts far away from the sun. The sector you appear in is claimed as yours. If there's no one in your immediate vicinity you will be put into forced sensor mode. Although you can turn sensor mode via interface, it still won't let you zoom in on your ships. The movement speed seems to be also increased in this phase of combat. Your goal now is usually to close onto your objectives so if you're not in time to complete them during this combat turn you can start close to them next time. Combat for a system is not designed to last one turn in most cases, but you can avoid getting carousels around a star by watching where the space is being claimed. As soon as you have hostiles on your sensors, the game lets you zoom in out of sensor mode so you can happily pew-pew.