Regarding this suggested crack, do I have to accept it as valid?
The code there is not valid 6502 machine code because it uses an opcode that doesn't exist (and won't work / crash on CMOS variants of the 6502 processor family). But it works correctly on all NMOS variants, and there wasn't ever an unmodified C64 shipped with a CMOS processor. So it will in practice only crash on systems like the C64 DTV (first generation) or a C64 equipped with a "turbo" extension like the SuperCPU that includes a CMOS processor.
Reject it: It isn't valid 6502 code any more, will crash on an "accelerated" C64 and some emulated systems, and if a crack like this would be allowed, 6502 code targetting the C64 could often be "trivially" cracked.
Accept it: The code targets the C64 and will work correctly on any real machine that wasn't modified/extended. As "undefined behavior" is explicitly allowed here as long as the actual behavior is the expected one, this is a valid crack.
So, what do you think? Do you have additional arguments for accepting/rejecting this crack?