I have not disassembled the production RP2350 rom (last i looked at it was ~year before release) so i do not know if it does indeed check nCS1 address space, but even if it did, which of the 4 possible pins would it configure as nCS1? surely trying all would be dangerous (possibly breaking customer logic attached to this pins that does not expect them to wiggle at boot). This seems like it could break customer designs so I would be worried if this did happen. I could be wrong. You can read out the rom with a debugger and look, of course :)
FLASH_DEVINFO (in OTP) has fields to set how large M1 is and what nCS pin to use for it. It's off by default.
Yes, you still have to write to OTP - but a few bits of configuration is a lot easier than an entire init routine.
ok, sorry, was trying to reply with regard to "no otp" as i understood your comment. i see i misunderstood. then yes it'll work