Do casual native English speakers understand the word unsaint as the opposite to the word saint? Or do they interpret it as something different or even don't understand it at all?
You can have a look at the Ngram, if you want. It shows that unsaint has been rarely used over the past century, while unsaintly has had some steady use.
If you asked me for an opposite of the word saint, I'd probably suggest sinner. I usually think of saint as a noun first, not a verb, and the expression saints and sinners gets used more often than unsaint and unsaintly combined.
"Unsaint" sounds like it wants to be a verb, but "saint" is not used as a verb in English. The verb for this process in Catholicism is "beatify" or "canonize" which would imply that the opposite should be "debeatify" or "decanonize" but I am not sure if this has ever happened. However, the dictionary does list "unsaint" as a verb and I think it would be understood in context.