Smooth-on makes a product called rebound 25 which is very good and used by lots of people for their silicon molds. It stretches very good and retains it's shape without the need for too many keys. Your registration keys are just something that will allow your silicon mold to keep its shape within the support shell (mother mold) during the casting process. Without the use of keys the silicon mold can shift during the rotocasting and once the resin begins to cure (harden) you'll end up with flat spots or dents from where it has pulled away. I mix up my silicon and pour small even amounts into ice cube trays in order to create my mold keys, I've also seen it done by using the small amounts of silicone leftover in the mixing cups once cured. I choose the cubes over the cups as I prefer the uniformity.
Support shell: This can be made very cheap and fast by using plaster bandages. A little more expensive option is the fiberglass, while an even more expensive option being epoxy putty's. This is all about personal preference and skill set. I use whatever I have laying around but can say the fiberglass is preferred.
Lastly, your base, model, helmet, whatever you want to call it needs to be as close to perfect as you can get it. Once you think it's there, take a 2 day break from it and then look at it once more. Any imperfections will need to be dealt with before you mold as the mold will just replicate them. This is why you see people using primer on their props before molding. They're looking for imperfections that must be addressed as doing the hard work now means you can run cast after cast with minimal issues.