Is it possible to bring a newer film simulation to an older camera?

Nobody knows yet. Exactly how film simulations work in the image processing pipeline and if/how they depend on the image processor is not understood.

Is it possible to downgrade the firmware on a body/lens?

Yes, it's as simple as modifying a few bytes in the firmware header. There's some risk involved in downgrading because the data non-volatile memory is not gauranteed to be backwards-compatible, but so far nobody has reported any issues.

You can use the web-based firmware patcher to modify the version of firmware files: https://fujihack.org/patcher/

Is it possible to run firmware for X camera on Y camera?

This is probably impossible between any two Fuji cameras. Slight differences like button mapping and minor hardware differences would likely cause major issues if not brick the camera.

Liveview overlays?

Probably easy to implement.