Yes, but the logic would be the same were it ten, twenty or two characters.
a) If it's a mandatory field, then an error message should be retrieved when the requirement isn't met.
b) ref. above
Maybe, maybe not. Has this issue come up for you before? If it were my site, I'd try and make my site as user-friendly, simple, and accessible as possible and avoid restrictions where they're not needed. If the scenario you describe is a realistic issue, meaning it's currently happening on a massive scale, then some sort of precaution should be taken. If you get the odd weird message every now and then, however, then I'd not do any intervention of any kind.
Yes, but it was clear and concise, wasn't it?