to mount your fs read/write, you do a mount /dev/sd?a /, or simply mount/

However, you probably don't want to - the only reason for going single-user is
to fsck.  Having fsck'ed with the fs mounted read-only, you do an exit to boot
multi-user, which will mount everything read/write.

(2) You can't run X-win in single-user, AFAIK.  To run it, type startx.
You might well need to do a ldconfig -m /usr/lib/X11R6 first, and set
your path to include things like /usr/X11R6/bin (possibly that line above
should say /usr/X11R6/lin)

Jules