I see you've overshot the decal with 2-3 coats of Tamiya clear. I'd try to poke tiny perferations into the decal, then lay some more of your clear on it, maybe use an eyedropper. Then wick up the excess with the corner of a paper towel or tissue. The silvering is beneath your decal, not above. If that top coat of clear dried, nothing you put atop it will penetrate. You will have to somehow get your clear
under the decal.
To lessen your silvering next time, make sure you have a very glossy surface to work on, or have plenty of MicroSol to lay the decal on. I personally have tried various methods, and have found that laying Future down until a good gloss surface shows is the best approach. It seals the colors and protects them from handling.