This is only a mathimatical projection of how things can go. The release date of 5 march isn't announced official (and think it will be posponed again). I just got that date out of an interview.
I can't follow your reasons saying that a demo can't come out before the betatester are done with it. To me a demo is always a beta test of a small part of the game with manny testers <img src="/ubbthreads/images/graemlins/badsmile2.gif" alt="" /> . I am shure that when the demo is release, a few people will experience problems and report them back to larian so that in the final game all bugs will hopefully be gone(programmers truth: A programmer can nerver kill all bugs. programmers truth: bugs are just extra gamplay option)when they release it.