I'd suggest you rent the two Zeiss lenses and take them for a test drive before you buy them and modify your camera.
I don't think you'll find all that huge a diff between images from the 21mm and from your 10-22mm. Or between your 100/2.8 USM macro and the 100 ZE Makro (except that the latter has f2.0).
The Zeiss lenses won't dim down the viewfinder due to the aperture, the way some manual focus lenses will. The reason is that the ZE lenses use an aperture control very similar to what all Canon lenses (including the MP-E 65mm and Canon TS-E lenses) do. The lens is maintained wide open until the instant the photo is taken, when it momentarily stops down.
In contrast, some manual focus lenses with fully manual aperture control too - such as the Rokinon/Samyang/Bower/ProOptic/Vivitar/etc. and any adapted vintage lens - do dim down the viewfinder whenever you manually stop the lens down prior to taking a shot. A vintage lens I use (Tamron SP 90mm f2.5 macro "Adaptall 2") makes for a pretty dim view when stopped down to f11 and combined with an extension tube!
Focus Confirmation will work on the Zeiss lenses and the Canon manual focus lenses, as well as on other manual focus and even adapted lenses so long as they are "chipped". But, again, Focus Confirmation begins to fail when the lens is stopped down to smaller apertures.
You can use Live View, too, though it's a slower process. If you set Exposure Simulation on, that can overcome dim views due to stopped down, manual aperture lenses. Using Live View to you'll find yourself zooming in to check focus, then back out to check composition.
Manual focus screens add features that old, manual focus cameras used to have, but modern AF cameras no longer do. Key amont these are a center "split image" focusing assist dot, sometimes surrounded by a "microdiaprism donut". These visually help you set manual focus. The center split image rangefinder dot is useful with shorter focal lengths, but will start to "black out" with longer (135mm and up). For longer focal lengths you have to switch to using the microdiaprisms (which is sort of like a whole bunch of very tiny split image rangefinders).
Previous post is correct... Likely if you add a manual focus screen to your 7D, your Spot Metering will no longer be usable. It's possible Partial Metering will be influenced, too. If using Evaluative Metering with the center AF point (or any active point(s) covered by the manual focus assist features), I'd be a bit concerned about the accuracy too, since that mode puts extra emphasis on the active AF point(s).
Oh, and the MP-E is a difficult lens to use for more than just it's manual focus. At the really high magnifications it's capable of, some of the problems are the same as when using a really long super telephoto: it can be tough to even find your subject and get them in the field of view. Just tightening the knobs on a tripod head can throw off your composition. Plus the slightest movement of the focus ring can throw things unrecognizably out of focus. Still, it's a lens that goes places no other lens can.
As an old fart who used manual focus cameras and lenses for a couple decades... I can tell you manual focus is nowhere near as fast as modern AF with a good AF lens and camera. And even when I was well practiced and my eyes and reflexes were a lot younger, I was nowhere near as accurate as modern AF, either. No one was. I have an old Thomas Mangelsen print that he shot back in the day with his Canon FD lenses and cameras.... and focus is slightly off (it's still a very nice print!)
A couple things I do sort of miss about the old days of manual focus, film, etc... Photography was a lot harder (try shooting everything at ISO 50 sometime) so there were far, far fewer "Craigs' List professionals" and you could still make a decent living selling stock or shooting weddings. And there was a whole lot less obsessing and whining about "sharpness". Hell, sometimes we were just happy to get the shot at all, and if it was critically sharp too that was a bonus.
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