Any clear coat will darken a flat painted surface, gloss coats more than flat clear coats. The clear flat will not return the apparent color of the flat paint to its original state, but it will lighten it.
There are several people, including Paul Boyer, who have used Future as a decal solution. While I have not tried it yet, the only danger I see is if the Future starts to cure before you get the decal positioned.
Do NOT use Future as a decal film to protect old decals by applying it to the decal sheet and allowing it to dry and cure. Cured Future is brittle, and it shrinks as it cures, too. Microscale makes a brushable decal film that works quite well to rescue old or fragile decals.