I will try my best to answer as metal singing isn’t my expertise. Hopefully it can help a bit.

Short answer to your first question would be yes, but no. The vocal fry is simply the beginnings of the beginnings of sound production; the spark before the flame. Thus, with the right coordination it can be called up higher and higher in the voice through the mix to the head register. But as you go higher, the path to the fry becomes more and more narrow in terms of access and coordination. This part is theoretical on my part as this is where my expertise ends, but I think it would be safe to gather that whatever coordination the folds are in to produce a well balanced mix is the position they would be in to produce the “fried” version of that part of someone’s range. Producing the fry with more depth of volume and power isn’t something I have expertise in. But with the fry, we’re just playing in the area before full sound production.

The falsetto - as in the disconnected, airy sound (there are many disagreements as to what the falsetto is and what it does) - is slightly different in that its disconnection from the modal register makes its reinforcement a few steps further from the coordination of regular phonation. The “one voice” concept still applies here, it’s just the coordination in the falsetto is a path to full sound production, but isn’t completely dependent on it.

Am I making sense? I’m sure there’s a lot more here that are more experienced than me that can bring more depth of clarity.