It sounds like you're doing most everything right. I had done some modifications and refinishing of bows before I started my first build, so I had a bit of experience with glass before that. It still blew me away how nasty the stuff is when you really start sanding it.
As for that dust, the most important things are the vacuum and the mask. You could also try using a fan behind you blowing dust either toward the vacuum if you have a dust hood, or out of the shop. I throw open the garage door and set up my sanding so that I'm blowing all the dust away from me, into the vacuum, and toward the open garage door. The good airflow has helped a lot.
Long sleeves will keep you from getting the itches, and I have an old shirt that I never wash and only wear when I'm sanding glass. After I'm done I take it off and hang it inside out in the shop. Any type of latex or nitrile disposable gloves will keep you form getting the itches on your hands, but they wont stop a bigger splinter. I've never had a problem with them though, and I like the fact that you dont have a set of gloves that may have glass dust inside them, when you're done, toss them and get a new pair next time.
I also use clamp forms, as I dont have enough storage space or a compresor for some air hose forms. What I would guess is that you didn't use enough glue or didn't spread it evenly. Just take your time. Next time I would tighten the clamps over your pressure strips, and then let the form sit for a while before putting it in the hot box. After a couple minutes some of the Smooth-On will have squeezed out of the glue joints and you can re-tighten the clamps. Repeat if you feel you need to, Smooth-On gives you plenty of working time. That will help you get good glue lines.
My one other tip is to try to cut as much of the limbs as possible, instead of grinding them down. Any fiberglass you take off in pieces is glass dust you aren't creating. I did my first bow with a hacksaw, taking the limbs down in about 3 inch chunks. It took about a solid week to do and I don't recommend it. Since I don't have a band saw, my solution was to use a cut-off wheel on a 4-1/2 inch angle head grinder. With two hands on the grinder and the blow clamped to some saw horses it is pretty easy and you can get really close to your lines with some practice. Saved me a lot of time, and I probably wouldn't still be building bows if a friend hadn't suggested that.
Hope this helps some, good luck with the build
-Nathan