Awesome. Not knowing that earlier, it would have been harder to answer questions obvioously, because I didn't know if you knewany of the basics. Didn't want to give you enough to be dangerous otherwise
Agreed. Again, just wanted to make sure you knew some basics
Understandable. The planning stage is the coolest part in my opinion 
I think you've got the right idea. I wouldn't cut your fiberglass jacket the way you described however. After your silicone layers have cured, you should clay up parting fins with registration keys for each section you intend on the jacket having. This way, the jacket sections can be bolted back together to contain your rubber properly. Without parting fins and registration keys, your jacket would never go back together as it was originally intended.
Somtimes I prefer not cutting the silicone on the form after it's been cured, but sometimes you have no choice. In a case like this with as large of a form as you intend on molding, it would probably be the best route. Certainly the fastest. Otherwise you would be applying silicone layers to clay parted sections, which would take forever If you don't mind a little bit of extra flashing to clean up, no bigee.
As far as sections and pieces go, this will definately have to have seperate cast pieces. For your sanity, as it would be next to impossible to make a casting out of the mold if it were molded complete, as-is. This is what I meant by saying "at LEAST a 2 part mold"
As MS said above, molding appenages seperately would save a lot of extra time sectioning off your jacket. What he described is like molding a figurine, just on a larger scale.
You'll most definatelyrun into issues with the protrusions off of the front of the helmet, and the visor of the helmet in relation to the face if you do the head as a whole. You may have to clay up a little more in the cheek areas to keep those undercuts to a minimum, while molding the visor seperately.
Here's a couple of quick examples ... the first one shows the body as the main support structure, allowing the neck to plug into the head, and the leg stems can plug into the feet (in red). The skirt in two halves. Brush, arms, etc are obvisously sperate.
The second example would be if you wanted to slush the rest of the body through the bottom of the feet (using the bottom of the feet as the opening to that part of the mold. You could then fill the inside of the body solid after slushing.
The arms could connect to the body via pegs, or dowels for support.
There are endless possibilities here. Many different ways of doing it. Hopefully this gives you something to work with It seems like a lot of pieces. But first time out, the more pieces the better I suppose
I'm sure someone else here will throw you some more suggestions 
On a side note, here is an awesome tutorial that might help you out. Granted, the form he is molding isn't 3 ft tall but it will give you s ome more ideas
I'm sure
http://www.danperezstudios.com/workshoppages/molding_casting.htm
FP
