I agree that the notwithstanding clause can't be used against S.3 rights, however you said more than that:

"Democratic rights can never be overridden. Neither can mobility rights. Ever. Under any circumstance. No legislature in Canada can ever suspend them. But other rights, such as religious rights, can."

Any legislature in Canada can suspend our democratic (voting) rights. And there could be perfectly valid reasons for doing so. But should that suspension be challenged in court, and the government lose its case, then it cannot resort to the notwithstanding clause to retain the suspension.

The same is true of mobility rights.