Well, the easy one to answer is that prcs can occupy only one side of the gestalt.
The question about the eldritch glaive invocation is quite a bit more complicated. On one hand, it acts in several respects like a weapon, but it's not a weapon, and there is a large number of things it cannot do as a result. On the other, it is weapon-like enough to make multiple attacks if your BAB is high enough, and you can apply a few feats to it as if it were a weapon. In the end, though, I'm going to say it's a gray area and rule against it in general.
In specific, though, I think that the number of things it can't do should be examined in detail, and a feat or two to ameliorate the difference should be provided. I say this because there is quite a number of "this is not a weapon" details behind eldritch glaive, and allowing it would be a great boost in raw damage for a martial character. As a result, how it works with Bladecraft should be carefully calibrated, if you want to go in that direction at all.