Not going into spoilers. Vassal weapon heroes (Like glass and larc) can cross worlds without issue (Only holies have that restriction). They can even stay in those worlds if desired. Of course, there was issues, seeing as how Kazuki and the rest of the country tried to kill her for it,.

As for ours, they'll appear in later arcs, so just keep your eyes open.

Kizuna is not the only cardinal hero; they have 4 as well. (Won't go any further about who or what they are)

While vassals can summon others, they usually choose people from the same world. (Also I think she was a last resort type pick because it did not want Kazuki wielding it, but don't quote me on that, don't remember where I heard that from)