Maths doesn't just sprout fully formed into being. All you can derive there is pure maths, which is fun but of limited use if you want to model the world. There are no mathematical proofs of RET, mathematical proofs are a specific thing which apply to the realm of numbers only.

What I expect you're talking about is mathematical formulae used in RET to make predictions or gain data. Those aren't mathematical proofs, just either evidence or deduction depending on application. Things like Newton's laws weren't proven mathematically, some guy with silly hair observed a few things and realised what factors were involved and drew conclusions. You can make mathematical proofs that draw on them, creating identities becomes pretty easy, but they're not mathematically proven so much as they are scientifically. (And yes, there's a difference, maths gives you 100% certainty so long as axioms hold and your proof-readers aren't missing something, science can get you 99.9999%).

As far as FET goes, there's maths. People just don't discuss it. Davis gave a formula from the infinite plane model (which is one of those FE models that uses gravity), JRowe have a formula for aether, it's pretty easy to derive a formula for denpressure...

But there are two basic kinds of maths in this context. One, the trivial. Two, the obscene.

You don't get much new information with the denpressure formula or infinite plane model, beyond the fact the relevant details could work. if you want, for dome models you can calculate the height of the dome if you want to make specific assumptions (the problem being trying to confirm those assumptions).

The other case is the obscenely hard. JRowe's aether formula I'm still not entirely convinced was meant seriously, but it's about what I'd expect the phenomenon he'd describe to look like. Think four dimensional partial differential equation. The kind of people that can derive brand new physical formulae are the top-tier geniuses of the world. Pick something you want the maths for under FET, and then try to figure out how that maths would actually get derived.

Chances are it's either something you could do easily, or that you wouldn't know where to start. Either there's no point in discussing it, or it's an absurd thing to demand.

Besides, most of the interesting parts of FET are in the mechanisms. Wait until you're happy with the mechanisms as potential explanations before you start pushing for more.