Bob, first off - sorry, I'm at work (and obviously in the December spirit!) and thus can't see your picture as our scanning at times stops posted pics. So I'll be purely commenting on what you've written untill I can see the pics from home tonight.
Silvering won't be "cured" by adding a clear layer over it, since it is achieved by trapping tiny air bubbles beneath the clear parts of the decals. Thick decals, Which will as such not be as susceptable to decal solvents and also don't "bend" as easy, may very well cause it. My best advice will be to allways apply it to as smooth a surface as possible. (Since you've used gloss with Future over it, that one goes out the window!) I allways use microsol on all decals and have since not had any problems with silvering. Don't know how that compares to the one you're using, but it's obtainable over here and works for me! Also, I've only airbrushed Future once and it came out somewhat of a mess, then got the advise to rather rbush it on and that's worked good for me! It tends to smooth out quite nicely. And as you say, add some toothpaste or Tamiya rubbing compound to the mix and you should get a very nice finish.
As for the gloss black... Maybe you should try using Tamiya's gloss black from their rattle-cans. I know the purists will shoot me for this! I tend to use their stuff quite a lot and it works very well for me. Build it up gradually with a couple of thin layers (over a base coat) and then a wettish layer. Some people tend to decant it and then use it with and airbrush - it's more economical and you've got more control, so that choice is there as well. I think trying to achieve a gloss finish (esp black!) with acrylics on a largish surface is downright near impossible.
I hate those kind of decisions... Good luck!