since I guess you misnumbered I'll go 1a and 1b:
1a) Idk if there's any sort of existing mechanic for this, but you have the idea maybe that all magic makes noticeable visual impressions on the Astral Plane, you could use the robe to maybe see and plot a course based on those impressions, like a ranger following tracks.
1b) Yes, that honestly sounds super OP to me as well. I was thinking that the longer you spend in the Astral Plane, the higher your chances of getting "marked" by the plane. Marked individuals have a very small chance of being attacked by an eldritch horror from the plane. The longer that the mark lasts, the more energy it accumulates, attracting more horrors, larger horrors, and making attacks more frequent. The worst part of it is that the mark is invisible and undetectable except to native dwellers of the Astral plane - most of which will be too busy trying to consume you to answer your questions.
2) I would say definitely not. Possibly you could go on a quest to upgrade it into (for example) a robe of skulls and stars that allows you to traverse not only the astral plane, but the plane of negative energy. But the plane of negative energy is even less hospitable than the astral plane - so you'd better not go without protection.
3) No. Not in the item description, and would definitely make it incredibly OP.
4) Sure. I'd say it's just like normal combat, unless you're fighting a being of the Astral plane, in which case they'd have some kind of home field advantage like etherealness or increased strength (or possibly disadvantage).
5) They would probably be able to talk and see them, but be unable to physically interact with them
6) Probably? see the part about astral markings though. If you want to live outside your natural plane of existence forever, there's always a price to pay.
7) Nope. I think of Astral projections as just that - projections. It's someone's mind, not someone's body, so giving them something will only give them a projection of something - they'll remember getting it from you, they'll be able to describe it maybe, but they won't actually have it physically.