Necronaut, this is great!
I first jumped into fiberglassing back in October when reinforcing WOF-templated gauntlets and helmet. I had a really tough time getting the stuff to stick into crevices - it seems to contract, or at least flex from the heat, a bit while curing. That makes it pull out of any tight space it's been wedged into, even if totally flush, wetted with resin, and without air bubbles. I decided the parts didn't need full reinforcement - they're just for show - but if you have the same problem and develop any tricks, let me know!
I did find a few tricks, through experimentation or Google searches, to combat the air bubble & mat overhang issues. I dealt with bubbles by "stabbing" at them with the bristles straight-on, like you suggested. That seemed to force the air out, and not just to the space around the bubble, but out through the mat. On relatively flat surfaces, this worked great - so "brushing" sideways is not just bad because it makes a hairy mess, it also tends to pull the mat and create air bubbles (or at least leave those that are already there).
The mat overhang I actually found useful, as it was a real pain if I tried to trim close to the part's actual size and wound up short after the resin went on. Grinding mat with a Dremel, minus resin, isn't exactly ideal, though - it gets wrapped around the shaft and can be a real pain to remove. So I just applied resin at least 1/2" (~1.5cm) past the edge of the part. After curing, I could Dremel the excess off without worrying about fiber strands getting into my tools. Not ideal if you have a part that is thicker at the very edge than in the middle, but if you're reinforcing a part of uniform thickness, it worked well for me.
And on the note of gauntlet darts, Chewiepal came up with some that are really good approximations for those on a budget:
http://www.thedentedhelmet.com/f25/cheap-darts-gauntlets-ala-chewiepal-20532/
As a beginner myself, I'll be watching this thread big time. Thanks!