I always prime everything before painting. Doing that has shown me on multiple occasions when I missed cleaning up a parting line or filling an ejector pin mark so I can get any strays cleaned up before the final paint goes on.
Since I'm using almost all MRP lacquers at this point, I don't usually bother with a clearcoat before decals because almost all of the MRP paints are semi-gloss and accept decals really well without getting silvering. I clearcoat after decals to protect them from my water-based Flory washes that I use for weathering...usually using either Semi Gloss or Semi Matte clearcoat, depending on how much "staining" I want from the wash. After that's done, I put on a final clearcoat to seal everything in and what sheen I select is based on what the real-world object looks like. Really like the MRP Semi Matte clearcoat for cockpits because that's the sheen they have in real life.
Its good to come to places like this and get advice, but try not to take our advice as an "always do this" or "always do that" set of absolutes. Try the various pieces of advice out and see what works best for you and your building style. I've been building models for a while and am still constantly learning and trying new techniques on my builds...its what keeps it interesting.