I can answer the first two
#1- No it's not too late. The future is needed to help keep the decals from silvering. This leads right into question 2.
#2- The white over spray should be done last. The normal steps are: Paint, Gloss (future), Decal, Flat, Weather.
My guess for the white is to put a few extra coats of flat over the decals so there is a bit of a barrier between the overcoat and the decals/paint then after spraying the white you can do some light sanding to remove some of the white to give it a worn appearance.
Wayners
Go Eagles! 334th Fighter Squadron
Me and my F-4E
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