DkCHoPSUEY

To the best of my knowledge and based on my personal experience, both primers are enamel-based. Lacquers have always worked fine for me over either one. As far as which one would be preferable, Krylon Indoor/Outdoor is quite a bit smoother than Rustoleum automotive primer and easier to control (read less paint comes out, it builds up slower, and it lays down nicely without much texture). I have tried Rustolem Painter's Touch primer and this brand is pretty close to Krylon.  The best thing about these brands is the price and availability.

No, they won't have any sheen with matte top coat.

Example silver become a gray with bit of flake.

The easiest way, if you want might actually be to use a gold gundam paint marker for the gold since it can be a bit easier to control for newer modelers than a handbrush in small areas.

Personally I always use a black undercoat with metallics. Gives them a nice depth IMO. Your mileage may vary, of course, and it depends how thick your metallic coat is. Some metallic paints (eg. Alclad) actually require a basecoat of gloss black, but I don't know about Mr Color. It couldn't hurt though!

Gold doesn't rust! It might tarnish a bit, but there shouldn't be any corrosion. I don't know where you are in the world, but if you can get hold of them, Citadel (Games Workshop) do some very nice metallics. They keep changing the names, but they used to do a "Shining Gold" that would match that tarnished look very well. EDIT: The new name is "Gehenna's Gold" apparently. Water-based acrylic, very nice to use, brushes on very well.

The musket looks simple - the parts that need to be gold are all raised, so should be easy to paint freehand. I wouldn't bother masking them. Basecoat of black, followed by the gold. I can't quite see from your linked pictures what bits on the head need to be gold, but it would depend on the part breakdown. Personally I'd try to keep masking to a minimum, but it's not always easy.