Do a loop selection on the mirror seam, enter, for the scale axis of the mirror plan a 0 value, suppress half a symmetry, redo your symmetry (they will now be connected together, select both, and weld both.
It's in french, but perhaps the images will help you (or use the google translation, down the page)
http://www.eovia3d.net/showthread.php?t=4561
Vagula has given the good answer for the second question
