Not really, most OS for phones are android based anyway. Having the choice of an ungoogled android is always a plus, much like many people just want an ungoogled chrome
This is unenforceable really. Android is a monolithic OS, it's very architecture would have to be reengineered, and then phone manufacturers would have to design their system drivers to be independent and distributable.
Windows is modular, this comes from the standard BIOS that came out of the bios wars in the 80's. Having a standard hardware interface enables this.
No it's not, just mandate unlocked or user-unlockable bootloaders, the open source Android dev community will do the rest. There would be degoogled custom ROMs and full linux (eg Ubuntu touch, postmarketOS, etc.) ports for every reasonably popular device within months.
Well that would be nice of course, but won't ever happen because manufacturers are all over the world and there's no global law or regulation that could enforce that as a requirement.
What I suggested is still the current best option consumers have.
Globally, it's impossible to force a company to do anything. But, with key markets forcing manufactures to comply, most companies decide to go the "well, i'll make it so for everyone then" route, as it can be cheaper in the long run.