These tiles are mesh-backed, and at least when I hold it in my hand I don't see a way they could move relative to each other, unless the mesh buckled.
Yeah, me too. I found that setting mosaic tile was more difficult that it first seemed.
You also might want to get some sandpaper and some wide masking tape.
I always set wall tiles without any additive in the thin set. I just used Mapei Kerabond Grey without the Keralastic additive. I found that the thin set would remain soft enough to remove easily with sandpaper for quite a while after setting the tiles.
So, if you get any thin set on the edges of the tiles, or any comes oozing out between the tiles, you can remove it easily by just folding a piece of sandpaper in half and running it up and down the grout line to remove the thin set and clean the edges of the tiles.
Mastic provides good "grab" when you set tiles so the tile don't move. However, tiles set with thin set are more prone to sliding down the wall. Since you can't support the mosaic tiles from below, I'd mark a line where you want the top edge of the tile to be and mask off the area you want to spread thin set over. Spread your thin set over 3 or 4 lineal feet of that counter and pull the masking tape off. Set your mosaic tiles along your pencil line and hold the top row of tiles up with masking tape so they "hang down" to the top of the counter. Otherwise there's a risk the tiles will slide down the wall.