Thanks for posting!
Regarding the decal silvering, applying a flat coat will not solve your problem. The silvering occurs beneath the decal. It's the result of applying a decal over a flat surface. Flat surfaces are pebbly at the microscopic level -- that's what makes them matte. However, if you cover it, they reflect light to the underside of the decal since the decal cannot conform to the microscopic pebbly surface -- causing the silvering. To avoid this, you have to apply decals to a glossy surface. Glossy surfaces are smooth and the decal adheres to it completely, avoiding any light reflection below the decal.
Your options now are to either carefully paint over the silvered decal film or to carefully pry off the decals, save them. Then apply a gloss coat to their former position and reapply the decals. If you need replacement decals, give out a shout. I and many others have tons of spares.
Or just chalk it up to a lesson learned and leave it as is. Only other modelers would ever notice the silvering anyway.
As you're just starting, may I suggest Kalmbach's "Modeling Realistic Tanks and Artillery: An Illustrated Guide " by Mike Ashley? Very useful instruction and great pictures to explain everything. Highly recommended and easily available at most hobby shops.
One last thing: you should consider your camera to be a tool. Photos allow you to spot mistakes in order to correct them. Because you're used to looking at your actual 3-dimensional model for hours on end, your brain misses mistakes that are in your build. I photograph my models in great light. Looking at the images on a screen, I can spot areas that need attention that I couldn't if I was just looking at my model. It's an old figure modeler's trick.
Best of luck to you!