post pictures of the room and your choices already please

If it was me, I would centre it and only silicone the back. I understand your concerns about water going down the side. Realistically, though, as you will only be washing your hands and maybe teeth in it, there is unlikely to be much water gushing about. To detract from the fact there is not as much space on either side of sink, I would have carpenter either wall mount or put legs on the cabinet so there is space underneath and the unit doesn't look too chunky!

I would paint the ceiling white.

Shamelessly bumping my own post- any of you super houzzers out the have any suggestions please? A mid grey perhaps or take the blue or yellow from tiles? Thanking you....