I'm probably missing something, but "First assume there's an abstract wavefunction and see what happens if everything is entangled" seems to me like a classic example of begging the question.
It's traditional to derive equations and functions from higher principles, not to work back from them.
You can't assume there's a wavefunction, because it's a metaphysical position. You're literally assuming an entire mathematical apparatus just sort of exists for no reason, in some wholly unspecified way, in an unspecified medium, with completely unknown properties.
By starting with an abstract premise and asking "let's work out what would happen?", any useful and surprising results tell us connections that we didn't see until we worked them out. If those results match the real world, we learned about the connection between the real world and a plausible abstract underlying structure.
This is a useful method how to derive equations and functions from other principles and empirical observations. You can think of it as a search strategy, to find simpler explanations and mechanisms that produce the complex consequences we observe in real life.
A lot of quantum theory was figured out this way, which has proved very useful in practice. Much of our modern technology depends on it.
You can establish the wave function and entanglement by doing experiments unrelated to gravity, so if gravity comes out of them that's still fairly impressive. (I am not sure if it actually does.) You are right that making assumptions and reasoning from them does not itself make your conclusions true, but it does change the "input port" where you have to plug in your empirical observations to some proposition hopefully more easily demonstrated.
It's traditional to derive equations and functions from higher principles, not to work back from them.
You can't assume there's a wavefunction, because it's a metaphysical position. You're literally assuming an entire mathematical apparatus just sort of exists for no reason, in some wholly unspecified way, in an unspecified medium, with completely unknown properties.