Hi Scott and welcome to the zoo.
The answers to some of your quesitons will be based on opinions, no right or wrong. So here are my opinions:
1) Personally, I think preshading has its place for certain schemes where heavy wear is to be represented but I would not use the technique myself for natural metal finish (NMF) representation. If you do chose to do it though, I have only seen it done with an airbrush but I suppose it would be possible to do it with a hairy stick (brush). You'd want to feather the edges out though so drybrushing the edges might work oruse a Q-Tip. Airbrush would be best though.
2) It's almost a given these days to gloss coat before applying decals. This alolows the decals to adhere better and lessens the cahnce of "silvering". If you're using something like Alclad metallizer - never used the stuff myself - I'm not sure what the protocol is before applying decals. Maybe someone else will chime in on that one
3) Dullcoat after applying the decals does seal them and also returns the paint work to a more realistic flat finish for painted aircraft. Coating over the decals also results in a more uniform finish between paint and decal. Again for a NMF, this will depend on the final look you want to acheive. You can also seal the decals with another gloss coat if you want a shinier finished look. I personally like my models to look like they're in a war and not in an airshow so I tend toward the duller, worn look.
4) As I said, I like the weathered look but the key is to be very subtle with this because it's easy to overdo it. For exhaust, dust, and dirt stains, I tend to use chalk pastels as these not only look realistic but you can slowly build the effect from almost invisible to the desired intensity. In cockpits and wheel wells, I like to use a very thin wash of black/brown oils to make highlights stand out and simulate dirt and grease. Carefully done, this technique makes for great oil stains as well.
Good luck and looking forward to seeing your stuff! Here are a couple of samples: