1. I would stick to 26 or 24 awg, as many strands as you can get. In general, the more strands, the more flexible.
2. Silver plated copper is nor really any less flexible than regular copper, with the caveat that most cheaper silver plated copper wire is sheathed in teflon or pvc, both of which you want to avoid if you really want flexibility.
3. Silicone or a specially designed polyethylene will be the most flexible. Problem is that the better silicone wire is specialty wire for stuff like robotics and is more expensive per foot than some audiophile stuff. Toxic Cables had a batch of OCC copper in polyethylene that was so flexible, I mistook it for silicone wire. PM him to see if he has any for sale. You won't find something more flexible or limp on the market. The caveat with this, is that it is harder to work with because of its flexibility, and there is some grab to the polyethylene that makes sleeving with something soft nearly impossible. In use, without sleeving, it was too flexible for me so I ended up putting a 1/8" sleeve of techflex around it and then putting nylon multifilament on top of that. Still super light, very flexible when you need to bend it, but lays very neatly now in a large coil. But if a specialty wire isn't possible, stick to polyethylene. Mogami and Canare both use polyethylene, but it is a harder variety and despite some decent stranding and gauge numbers, they aren't as flexible as they could be (durability was top priority for these companies). Doublehelixcables, Toxic Cables, and Plussounds come immediately to mind for high quality, reasonable cost wire that is nice and flexible (and audiophile approved).
4. Nylon multifilament or paracord is the way to go for soft, flexible sleeving. Not many other options unless you can find cotton or silk, which is very hard to do. Any of the Techflex type stuff is going to add stiffness.
Hope that helps! Have fun with it.