Charles,
I think some of it depends on the weight of the camera rig also. I have several lever release clamps from RRS that I use on everything from a big sturdy tripod to several monopods used with big glass (400mm f2.8 size). Have never had a problem with the lever release. One was a bit loose, and I sent it back to RRS for tightening, which they did, and it works great. The design is such that should the lever flip open, the plate will not just fallout out. They have stops built into them to prevent that, and they work. If the lever is completely opened, then things can fall out, but I have yet to have that happen and my stuff gets carted around into some tough settings at times.
I do agree with Bob about considering the screw clamp, especially if you may have Arca Swiss dovetail plates that are older, heavily used, or not made by RRS, such as Kirks and others. There could be some play with the RRS lever release, and that gets completely mitigated by using the screw mount instead. I may opt for a screw mount for my next version, simply for that security, but so far, the lever releases have been solid and secure and a joy to use.
As for the spirit level....honestly, I have not used it much, but then most of my shooting has not required it, and I try to get my comps straight by eye for action shooting. If I were doing panos or architecture types shots, I would look toward using the levels more, and think the separate axis types are better than the simple bulls-eye bubble. The simple bubble will get you pretty close quickly and that may be enough, depending upon what you are shooting and how.
LJ