Hello, Cherokeedog, and welcome to the forums!
As far as I know, the off-line mode will just be an exploration-centered experience starting off. I doubt there will be combat, as that would mean developing a competent ai, which would take time, which the devs don't really have to spend on anything other than the multiplayer right now. Further down the line, who knows? But at the moment, I believe it's just an exploration experience. And from playing the proto right now, it's a really good one, and that's with just three moons. There will be more planets in the future, developing this into a fairly large solar system. I don't doubt you'll be able to fly any ship that you would be able to in the multiplayer, so in that case you can get valuable flight experience.
The engine at this point I could create an entire galaxy, and there are tech demos to prove so. In fact, I believe the skybox was created by an engine-created galaxy. However, in terms of the multiplayer, the focus is just on a single system, mostly because as a combat-orientated game, we can't have people wandering off and not fighting. In the single player, down the line, who knows?
As to the last point, yes, you can. Exactly what depends on what the devs have time to implement. Right now there's an NPC hauler that will eventually haul resources to and from bases, so escorts will be essential. Players will be able to fly carriers, which will re-supply smaller ships, so I'd imagine keeping the carrier supplied and in a strategic position will be in-game. I'm not sure what the plan is regarding sensors/stealth/spying, a dev will have to enlighten me, however all of those afaik are going to be implemented somehow.
Hopefully that answers some of your questions.