Arca Swiss dovetail clamps and rails: you can indeed slide the rail, camera bracket, or lens foot back and forth /side to side a small amount. When I mount my 400mm f/5.6 with teleconverter, the Arca plate attached to the lens' tripod ring is in a slightly different position than when I mount just the 400mm lens, to balance properly. Panorama rigs take advantage of the AS dovetail design - you can mount the camera at the end of a long rail, put the rail's other end into an AS clamp, then move the rail back and forth in its clamp to the proper no-parallax point for whatever lens is mounted on the camera at the time. One rail can service many lenses. For macro, it may be more efficient to get a macro-specific rail that has a movable top clamp.
Screw vs. lever: it's a matter of preference. I have both. If you have a motley collection of rails, brackets, lens feet by many manufacturers, a few lever clamps may not conveniently fit all. I have a screw mount on two tripods, in one case because I wanted a very compact (Sunfotoway disc-shaped) clamp that the tripod legs could be reversed to fit around, in the other case because I bought a kit to adapt my Manfrotto geared head to Arca-Swiss clamp and the only choice was a screw clamp. I tried a lever clamp on the third tripod/head, Arca-Swiss brand lever clamp on a Arca-Swiss Z1 ball head, and like it too. It is very secure for the Kirk and Hejnar brackets, feet, and rails I use, and cannot be readily adjusted to fit both those items and a small Sunwayfoto rail. I don't use that Sunwayfoto rail on that tripod, I use it on the smaller travel tripod, which is the tripod I use for the compact camera that the rail is intended for. The safety feature works well, it is a two step process to remove the camera/lens/rail.
Length of the AS clamp: The longer the jaws, the more friction exerted. Don't put a heavy rig on 1" long clamp. My clamps have jaws of 2" to 3". 3" should hold a supertelephoto.
