Is the spring grip very strong? & w/ rectangular swiss not square?
The spring is sturdy, but you can actually lock the mechanism in place using the red knob. In the photos, it seems to show using the red knob to rotate the grip. While you can do that (once the grip is locked) it will only rotate in one direction. I find it easier to just rotate the grip directly.
Will it sit well into an Arca type tripod receiver?
Yes with a standard arca swiss base plate attached to the bottom.