I'm by no means a pro, but hopefully my comments can help out. I'll try to go through what you have one by one.
Using flash in the dark hall - I can't exactly see your makeup in that photo, but I can tell you whoever shot that photo pointed the flash directly at you and didn't diffuse it. You can see how defined and large the shadow casted by the flash is behind you two. Usually you want a soft light so that you can see the details but not blow out anything. You can do that by bouncing flash off the ceiling but at this location, the ceiling is too high so you'll need a bounce card or diffuser. That will soften the light instead of the harsh direct light.
Cosplayer - The 1st photo inside is nice, maybe a little too much empty space in the top portion. Maybe if she had held the big gun on her shoulder, it would have filled it better.
Outside photos - Yeah, you want to avoid harsh sunlight overhead, your subject will always have to squint and there's just too much light. If possible, move them to a shady area, there should be enough natural light.
As for why it's blurry - What aperture/shutter speed were you using?
For the cosplayer in the dress - It's a good photo for showing the entire cosplay, but I don't like how the light is casted on her face. This would have been a good place to use fill flash to light up her face to match the rest of the lighting.
To make the photo more interesting, try different angles. Straight on from the front is usually plain, it gets you a full view of the cosplay but that's about it unless the cosplayer has an awesome pose. But if you're blocking people, there's not much you can do if you're trying to be quick.
Your favorite picture - I pretty much agree with what you thought about it.
The photo you used flash - It's a good shot. Is this the popup flash straight on? Try diffusing it and/or getting a flash that you can angle the lighting.
As for photoshoots - How did you advertise yourself for photoshoots prior to the convention? And if there were no photoshoots at all, maybe the cosplayers weren't expecting to have photoshoots there while the convention was having construction?
At the convention, if you want to shoot more photos of a certain cosplayers, just ask them. Worst that will happen is they say no, and you move on. As for taking off badges, backpacks, etc., again just ask and explain that it will make for a better shot. Unless they are in a rush for a panel or something, most cosplayers I've seen will drop their stuff and strike a pose if asked nicely.
Again, I'm not a pro so maybe I'm off on what I said. One thing that I do though, is compare my shots of cosplayers with other photographers's shots of the same cosplayer and examine the differences. You'll see what works and what doesn't and how you should approach the shot next time. Seeing professional photographers's composition is very helpful.
whew, that was long lol
You can call me Jono
San Diego Comic-Con, Otakon, Dragon*Con