I don't think so, in practice since atomics (on single-core systems especially) and floats can easily be emulated. If larger, more-incompatible extensions become commonplace, possibly.
I mean, in the very least the proprietary ISA vendors provide the service of selecting what extensions are added. If you think too many extensions will take down RISC-V, that seems to have value.
RISC-V will do the same thing. That's what profiles and even further platforms will provide. Most software (the large investment) will target these profiles.
Not following the profiles will be possible but will cause issues.
Not since RISC-V Profiles[0].
This is going to confuse the hell out of the potential customer base and it would be a crying shame, wasting a golden opportunity to get out of the oxygen-choking clutches of proprietary ISA vendors (who at this point are purely rent-seeking and add exactly zero actual economic value to the ecosystem).