They actually did do that. C was split into ZcfZcdZca, so you can choose a non-overlapping subset. It doesn't affect an RV32 non-F core anyway.

Sure can. Just change the encodings.

You thought RISC-V chips were compatible with each other beyond the basics? They're not. RISC-V is only a starting point for designing the ISA your chip will actually implement. Don't get me wrong - it's still beneficial that simple code works on many chips.

Practically you don't simultaneously want those overlapping encodings.