I suggest changing '$post[musername]' to simply '$post[username]' as musername also includes the bold and italic tags, making the quote dysfunction.
Other than that, works wonderfully on 3.6.4! Even though there is no template hook in postbit, adding it simply made no difference!
And of course, let us not forget the required worshiping *worships*. I had been dreaming of finding something exactly like this for a while now (and I am a WYSIWYG lover)!