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!

First image is the wall tile with the charcoal tile with a colour card for the vanity. Next image is the vanity the other 2 are floor tiles Many Thanks

my main question is.. is there an alternative floor anyone can suggest. i like rustic / wood, not high gloss.

I have recently tiled my bathroom. But I am stuck on colors. I was thinking navy sink unit and bath but is it too much any suggestions appreciated!