Ich is in the water, the rock, the sand, etc. Draining will have no effect on ridding of it. The only way to get rid of it in a DT is by starving them out essentially.
The problem with adding more fish to a qt is that they will be behind. Ich is not super serious. Yes it does kill fish but it is a lot slower moving than velvet or brook. You could wait till you start your treatment to get the fish so they are all on the same schedule.
Yes you can add coral and more rock while you are qt'ing, but your clock will restart. Anything wet(snails, hermits, corals, rock, live sand) technically needs quarantined. With coral though, the tomonts(sacs) can't attach to live flesh. The theronts(free swimming stage) though can be on them, as they only need a droplet of water.
To be 100% sure, you need to quarantine everything for a min of 72 days. There are many good stickies to read up on. It will save you a lot of money and a huge headache. Also, we are all here to help so ask us anything and we will give our best advice.