That i can answer without wiredly going out of temper. Sorry again endso and all who thought better of me, it took a walk through the forest to realize i should have just not posted before understanding...

"For a device to be covered by warranty, it must be running Android OS."

Is to be taken literally. It was stated during the community events that if you send it in, it must run android so the (industry) standard tools to check for faults work.
If you manage to reflash to android before sending in, all is fine. That is my information, not a legal expert though.

"F(x)tec cannot be responsible for any issues which arise from installing alternative operating systems."

Might make it necessary to trust FX to not try and pull legal strings to avoid service once they took the effort and checked if your device was tainted with alternate OS before. correct.
I have their word they will not bother once a sent in device runs android and will measure them on that.