If it were your forum, wouldn’t you have fixed it by now if it was an easy fix and there was no mitigating factor?
There is a factor we’re not privy to. I can accept that.
I hate to tell you but it is an easy fix. Simple RegEx expression in the validation code when posting replacing the offending characters with acceptable ones and viola. Fixed. It is a UTF8 <-> ANSI issue. Sanitize posts in existing code, and you will not invoke hardly any additional CPU cycle overhead. Nothing else would be affected. A few CPU cycles to find the offending characters and replace them in memory before committing the post to the database.
Example: wouldn’t would become wouldn’t Look at the character in the post you made. copy and paste your original wouldn’t to me, and copy and paste my wouldn’t as well. It is a slanting ‘ Straight ones work fine. “ is another one. Opening and closing quotes mess up. Straight ones (ASCII) work fine.