Your first part was irrelevant: if you cast away const, of course variable must have been const in first place. As other have already mentioned, casting itself is not an undefined behavior (and sometimes even have legitimate reasons, mentioned in my earlier post). On the other hand, trying to modify const variable though non-const pointer is undefined behavior.
P.S. if you downvoted to disagree with this (on my parent post), please provide an opposite example, because as a C programmer, I would really be interested to be proven wrong on this matter.
No, it could be mutable variable referenced/accessed (not sure of the right word here) in a function via either a reference-to-const or a pointer-to-const.
It seems highly non-intuitive to me that casting away constness works differently depending on whether the variable was defined const or not, but if that's the rules, that's the rules.
P.S. if you downvoted to disagree with this (on my parent post), please provide an opposite example, because as a C programmer, I would really be interested to be proven wrong on this matter.