What ever happened to Susan?

So I've been buying up Doctor Who DVD's from Amazon like they were going out of style.  I swear I'm keeping the economy going.  I  must be over 70.  My latest purchase was Dalek Invasion of Earth.  I always wanted to know what happened to Susan the granddaughter.  For the most part, even though Doctor Who is non linear wibley wobbley timey wimey, I can understand it.  But Susan is the fly in the ointment.  If she is there, it kind of messes with everything.  The general explanation of her being left behind because she was in love is good but sort of lacks and really screws with time lines.  Prime Directive, Anyone?  According to the cannon of the radio plays she has children and if you follow the books as well and accept that the Doctor is half Time Lord then she is a quarter Time Lord.  Why has he never gone back and visited her?  One of the things I love about the BBC DVD's is the extras.  On my copy of Dalek Invasion of Earth there is a production called What Happen To Susan?  It has a different explanation of where she went.  I don't believe that the BBC actually authorized this although they certainly utilized it.  I thought buying this DVD would clear things up but it only served to make me more confused.  It also cast the Doctor in a sort of bad light.  I wouldn't believe the things said of the Doctor.  Although I thoroughly enjoyed the DVD and expanding my horizons.  I think that is one of the best things about Doctor Who.  It makes you think outside the box that's bigger on the inside.  If the problem can't be solved you just haven't though of it properly.