Is this rear mount sufficient?

That'll hold but yes, you could crush your seat post tube tightening these type clamps down too much. The way I do mine is by using 2 of the rear mount blocks around the seat post tube so it can clamp it tightly but there's enough surface to prevent crushing, it's cheap but effective.

You could also put a shim between that muffler clamp and your seat post tube to spread the load and prevent crushing.

What I do with these type of mounts is fill it with JB stick and let it set up then sand it smooth. So far not issues with mine and they get torqued down pretty good. Hope this helps!