A lot to cover here, but I'll take a stab at it.
1. I don't water down base coats. Using a wet palette will keep your paints fluid enough to not blotch up on you. I showed my cheap wet palette in my project log. A Rubbermaid food container, a sponge, and a sheet of parchment paper is all that you need to make one. I painted for 20 years without one, I'll never go back. I tend to get base coats down in a single coat, though the color that you are using can sometimes require two coats. (Yellow is infamous for requiring more than one coat)
2. You will find that you don't have to water down the highlight paints as much using a wet palette. Some paints require a bit of thinning, though I don't do much thinning to be honest if the paint itself doesn't require it.
3. Washes work best if you don't water them down. I wipe just a little bit off before applying.
4. I prefer to shade only certain areas with washes, though it is not completely necessary. You can re-highlight after washing to get the shine back.
5. I still have my older GW metallics,but have found the newer bronze, brass metallics to be fairly watery to begin with. Again, I think that using a wet palette will solve the problem of needing to thin your paints. (This seems to be a reoccurring theme in your questions/painting problems)
I hope this helps!