My take on your questions:
1) I personally only prime on multi media (resin, PE & plastic) cockpits. If you are building straight OOB, no need to prime (unless you are using acrylics with poor adhesion). Otherwise just use the base color of Dark Gull Gray as your primer/base coat once you have the major sub assembies done.
2) if you are gonna prime the exterior(for the same reasons I listed above) gloss clear is not a wise idea. The whole idea of primer is to give some thing for the paint to adhere to. A gloss surface is much slicker than a flat surface, and provides less "tooth" for the paint to grab on to. Use something like Tamiya white Surface Primer, Gunze Mr Surfacer, Humbrol primer, or some similar product for better results. Especially on the white lower surfaces. White is more of a challenge to apply nicely in many cases.
F is for FIRE, That burns down the whole town!
U is for URANIUM... BOMBS!
N is for NO SURVIVORS...
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LSM