At first, after experiencing (but most likely not completely comprehending) the story, I thought that Ciel made Model XZHFLP, whilst I thought that (at first) it was Dr. Viel who made Model V. Then, after playing ZX Advent, I learned that Albert created(?) Model V.
Perfectly understandable. Albert as a character was not introduced before ZX Advent, nor were Model W's origins completely explained. It was only stated that it was derived from the remnants of Ragnarok, but how it got to be something capable of transforming others with the R.O.C.K. system was never explored, nor were the origins of Pandora and Prometheus, the self-proclaimed voices of Model W.
And I need some clearing up. While Aile/Vent were away, they left Models HFLP in a lab, and someone stole 'em?
Yes. And the Guardian science guy's name is Fleuve.
Also, why was Aile/Vent compatible with HFLP?
They weren't compatible with any model other than X, but X lets them double merge with any other biometal, so they can use their powers anyway.
Vent and Aile are among Albert's Chosen Ones, and for the last time, any
of Ciel's Biometals are compatible with any
Chosen One. Any notions to the contrary are complete fanon, and with the possible exception of Model A (being Albert's backup), there is no conceivable reason that such a restriction should exist at all.
Ciel's Biometals (Models X, Z, H, L, F, and P) are based off of her research on Model W. There can be no further restriction within Ciel's usage of the R.O.C.K. System beyond Model W's unless Ciel herself added it, and she has neither the reason nor the ability to do so. If she could modify R.O.C.K. System compatibility, she would never have limited her Biometals to those who share compatibility with Model W in the first place. Given that she is stuck within the same "pool" as Model W, limiting her Biometals any further would only serve to handicap them against the very threat they were designed to counter, and for what benefit? None. The whole idea is a fan-based assumption, and a grossly illogical one at that.
"...the ones with the potential to use Model X and Z, are the same ones with the potential to unleash Model W. As a chosen one, you have the power to save or destroy the world."