The stars (or, more likely, planets circling them) would be the sites of the "dream" experiences he had had (which, of course, were not really dreams, but rather his somnambulistic hyperspace travel); hence an established "contact" with those places which would unconsciously keep drawing his attention (with an attendant desire to physically return) to them.
As for Brown Jenkin -- he (it?) is the traditional witch's familiar, given to every witch who has signed herself away in the "Black Book" of legend and folklore; it acts as an intermediary between the powers she serves and the witch herself (or himself), as well as a sort of servant (though with an independent will of its own when appropriate).
The former of these is left perhaps a bit too nebulous, though. As for "plot holes" -- one of the most glaring is the likelihood of Keziah, considering her advanced knowledge and connection to such cosmic forces, not to mention her longevity and the awareness which would come with it, being frightened by a crucifix, and the fact that this fear is a little too facilely opportune to give Gilman a chance to (successfully) fight back.
Now, on this one, I have argued with Joshi a long time ago, though I don't think we ever came to an agreement. My point would be that, no matter how advanced, in the heat of an emotional moment, a person is likely to react on a very primitive level -- be affected by associations learned in earliest childhood, at least momentarily; in this case, long enough for Gilman to grasp the chain and use it. The association here, of course, being based on her growing up in a Puritan theocracy, and the associations the crucifix would have for someone of that background -- the early-instilled "fear of God" combined with a hatred/fear of the Catholic religion with its history of torture for those of other faiths, combined with the similar reaction of various Puritan movements from her childhood. It's a slender palliative for a rather glaring weak spot, but it is within reason that someone might react like that at such a moment, if only for the briefest of instants. Still, it does come across as the hackneyed "cross-and-vampire" idea, hence weakening the story at a crucial point.
(Of course, the Puritans themselves tended to disdain crucifixes worn as amulets, because of the associations with the "Popish" traditions of whom they were inexorable enemies, so the choice of this symbol, handled in this way, makes even the above rather more tenuous still.)
As for "The Silver Key" -- that's a pity, as I think it is one of his most eloquent and mystical statements, and the time-travel element is handled with great skill and originality (rather marred, despite his best efforts, by having to follow at least the skeletal outline of Price's own attempt at a sequel, "Lord of Illusions", which became "Through the Gates of the Silver Key"). It is also a story which I think plays on a lot of levels... but it does have a bit too much of the philosophical and not enough of the "story" about it for many....