italian77
1. How many coats of primer should I use? My first coat of primer dried very fast and had orange peel so I sanded it but it took parts of that coat off.
One, typically. Unless you see imperfections. Then it is putty/sand/whatever and reprime. The primer shouldn't be causing the imperfections, though! 
italian77
2. Should I add the retarder to the primer, or the paint, or both?
Just the paint, and don't use it with anything but Vallejo. They offer more than one retarder. One is for airbrush use and is called "Airbrush Flow Improver". If you have the product with the word 'retarder', it is meant for brush painting and will likely cause more harm than good in an airbrush. At least that has been my experience. The "Airbrush Flow Improver" works great though. I hope that's the one you have.
italian77
3.one I am done priming and I then paint my shell how long do I wait for it to cure before I can put the decal stencil on so it does not peel off the paint when I peel the decal off? I am concerned that I will get the paint looking good and then immediately ruin it once I peel the decal. Should I apply varnish over the base coat to keep the decal from peeling the paint off, but if I do that will the paint I spray through the stencil stick to the varnish?
It depends, with the products you're working with I'd allow at least overnight between coats, and a couple days for decals until you find out what you can get away it. Dry to the touch is a fair indication ok to for next step.
Steps are usually paint, clear coat gloss (but satin is ok) to give decals a smooth surface. Wait a day. Apply decals (and setting solution if needed). Wait until dry to the touch, then wait a little more. Clear coat again to protect decals and make the edges blend in.
I don't know why your primer is orange peeling, but that shouldn't be happening. There is a lot of info on orange peel online. Many of the top search engine results bring your right back here to FSM.
Hope some of this is helpful. Good luck and don't forget to have fun.