1- yes. You can use the HTML BBC tag. However, why? it's a bad idea for even the admin to do this regularly... and almost any formatting stuff that you need can be accomplished through the use of the BBC tags (or wysiwyg editor)
2- There are two or three mods that allow this to be done.
3- SEO URLs are completely useless for SEO. Search engines can scan standard forum urls just fine. There are a few mods which can be added to improve things like the description, etc... but any type of "optimization" is mostly wasted on the forums. Forums do not have static content, and thus are not well suited to "optimization" techniques. There are several large discussions about this in the chit-chat and building your forum boards.
4- actually, captcha is mostly useless at this point. There are "questions" built in to 2.x for registration... I don't think there are any mods to apply those to posting.. but by adding mods for stop forum spam and project honeypot, you can reduce your psma registrations that even get to post to near zero.