Slow down cowboy, you're overthinking this whole thing (I speak from experience, I posted something similar a while back).
Use your QT tank as not only a treatment area for sick fish, but as a holding tank to observe new fish. Alot of times you're going to see fish in the store and buy them (like most of us) right away. You add them to your big tank and suddenly some symptoms show up and BINGO! you've contaminated your show tank. If you use a QT tank, you can observe your fish and treat them before they go in to your show tank.
When you get any new additions, put them in your QT tank and observe them for about 21 days, that should be enough time for symptoms to appear. No problems? Then don't worry, just acclimate them to your show tank and you're ready. See symptoms? Jump into the disease forum here, state what you're observing and I guarantee you'll get some good advice.
1. Ten gallons is a large enough size, depending of course on the size of fish you purchase. Obviously a mature volitan lion isn't going to fit in there.
2. Salinity should be the same as your show tank. If you need to treat ich and go with hyposalinity, you can lower the salinity over a 48 hour period. Only do it if you need to.
3. Use pvc or some other inert material as shelter for the fish. Don't use live rock or put anything like coral in there. Most medicines will have an adverse effect on invertebrates etc. As far as rock goes, you don't want anything that can soak up medicine, thus the use of the pvc.
4. Best way to cycle the QT tank is to simply transfer some H20 from your show tank during a water change. It will save a lot of time and headache. If you start getting algae etc. throw some turbo snails in, but take them out if you need to add any chemicals (see point #2 above).
5. How long you leave a fish in there will be dependent on the disease, your treatment and the fish's progress. Can't worry about it until you see symptoms.
6. I wouldn't put a substrate in, medicines like copper can soak into the sand and it's a b***ch taking your sand out after a treatment. Just leave a glass bottom, that way you can use a syphon to clean out the solid waste.
Hope this helps. It's cool that you're thinking of all the alternatives, you're fish will appreciate your thoroughness!