Cherrypicking a little but to start it's not all that clear what type of Festum Kazuki and Soushi count as. Soushi IS a full-on human-Festum hybrid as much as any of the rest, remember it was his entire body that was assimilated, so his current self is a totally new reconstruction. He says his body is pretty much entirely unique so it might not have a perfect analogue with existing types, but he of course does have full on Festum-style rebirth as we see.
Kazuki was definitely in danger of dying but I think that was more due to overexerting himself in Sein than anything, since he did manage to successfully assimilate Aviator and that took some doing. But yeah with his permission the island's Mir completely turned him into a Festum hybrid too, to allow him to keep on fighting, forever if needed.
As far as Bollearios goes it helps to keep in mind Azazel-types are basically just walking Mir anyway. The name is just a technicality. So it took a new form, possibly to keep itself out of harm's way since the "Floater" appearance was pretty much a hollow disguise, and the NUN identified this giant Festum they didn't recognize as another Azazel-type. And yeah Kurusu is its Core, more than anything sort of mimicking Tatsumiya's style in the continuing blurring the lines between human and Festum.
Vagrant is assumption on my part, but I always thought they must have nabbed it relatively soon after Polaris was destroyed. Mir don't just become fully-formed monstrosities overnight, presumably they could have found a large enough crystal chunk and used Bartland's research along with all the Wadatsumi stuff to mold it in any way that Hester wanted.
The Prometheus/Wadatsumi Core was always attached to Jonathan, no matter where or when since he was a clone of it. Calling it a Gregory-type was just a mistaken assumption on everyone's part, they really are just harmless assimilated spirits and it looked like it fit right in with them.