Looking great!
About yellowed, old decals: I think Microscale makes a product called "Liquitape" or "Super film". (could be some other company, I forget.) Basically what it is, is solution that you brush onto the decal. When it dries it forms a clear film of decal over the original decal, adhering itself to it, so your decals don't crack and flake and disintegrate. I'm not sure if I'm being too clear - imagine a window completely duct taped on one side to prevent shattering from an explosion - that's the best analogy I can come up with.