Hi there,
Yes, the starch will interfere to some point since part of it's function keeps stains from getting as bad as they could. Paint, dye or ink will tend to sit on the surface of the starch and not bond with the fabric. If you can rinse the fabric first, you'll have a lot better experience.
Hmm, subtle. You might try something like one of the Lumiere paints that is not too far from the color of your fabric. The Lumiere paints have a shimmer/sparkle too them, and can be subtle but rich looking. The easiest way to use it as a stamp is to put a bit of paint onto a smooth surface like a piece of plexiglass and use a brayer to roll it off the surface onto the stamp. Or you can try using a small paintbrush to put the paint on the stamp. I also like DynaFlow paints for stamping. The one's Sarah recommended are good too. I don't aways like the coverage I get with a regular stamp pad, so I prefer to paint the ink onto the stamp. I'd also do some practice stamps before I put it on the actual quilt.
And do it on a padded surface. If you do it after layering, that would probably be enough padding. But I'd worry about the ink soaking through to the underside, so if you layer first, slip a piece of cardboard between the top layer and your batting before stamping.
You should be able to quilt though it once it's set. Usually the best way to do that is to let it dry and then press it from the back. Echo quilting might look really nice too, depending on your stamp.
Cherie