A little late to this.

You do not have to follow every last step listed. Nor do you have to use the products called out. Thats that persons preference.

Primer. Very often I simply use flat black paint. I like to use the black to show through the color to varying degrees to help with weathering. Often called black basing. If its a metallic finish, then I use gloss or semi gloss black depending on the effect I want.

Gloss coat. There are dozens of choices from Future to Tamiya to Alclad and on and on. And again, it will depend on what youre doing. I dont use it on metallic finishes.

I have never sealed decals before weathering. The decals represent painted on stuff on the real thing, so they should be as weathered and effected as the rest of the plane. Simply ensure you use Micro set or Solvaset to get the decals ON.

A dull coat goes on after all the decaling and weathering is done. You want to knock back the gloss and seal in what you have done. Again, there are tons of products to try.

As for your specific questions.

1. IMO, there are a lot more steps there than needed. As I outlined. But thats what *I* do.

2. Again, as I said, I dont use a varnish on metallic paints. But if I did, I would use Alclads.

3. Once the primer is set, its set. Yes, the next coat will bite into it, but it wont effect it in any noticeable way. You would have to heavily over spray the next coat to cause the primer to run. But again, you have to allow it to set. 24 hours minimum is usually suggested.