For the purposes of Dota and TF2 there is no way to make a plane double sided. You really shouldn't have any paper-thin geometry anywhere in the model, though...

As I said, it's modeled over the default backpack: http://cloud-2.steampowered.com/ugc/1135165172056349637/D9C5E199D0BF164C74C23357FCD5FB25E211C215/1024x578.resizedimage

Well if I understood correctly, it should just be a simple matter of hiding the head bodygroup. When you load up the in-game importer, there should be an option to hide the head geometry. The bip_head bone should work.

Could be that he's a mutant with the power to read peoples' minds. Who knows

I'd go ahead and post them. If you do it quickly enough, it will be obvious that you just happened to come up with the same idea at the same time. If you wait, it will look like you just copied their idea.

Nobody can really contest original work. If it's original, then it's original. As long as you didn't copy/paste anything, I think it's fine. If Valve hasn't added something that's been in the workshop for a while, and someone decides to remake it as a different model, I still think that's fair game.

Off the top of my head this issue is just hiding a bodygroup (the scouts backpack), but it doesn't seem like the importer allows for that option. I checked the scout model in the HLMV and it doesn't seem like he even has a bodygroup for his backpack. Are you sure there are any items that remove his backpack? I think there might be one item that does that, but it probably doesn't replace the backpack (it's probably just modeled over the backpack).

The entire model should be smooth shaded (not flat shaded), but in order for the model to not look like rubber you have to use smoothing groups (in blender these are called edgesplit modifiers). Select an edge that you want to appear "hard" and press ctrl+e > mark sharp. Then apply an edgesplit modifier to your model and set it so that it uses sharp edges. Google edgesplit modifier to see what I mean (I also explain how to use them in the TF2 video).

Lots of constructive criticism: Definitely stay away from pure colors (each part of the model looks like pure black, pure yellow, etc.). Also try to use hard edges otherwise the entire thing will look like it's made out of rubber. The part connecting the handle to the flail is very sickly looking. Perhaps you could make it a chain or something.

The glow is also not something very pronounced. It's just a material setting (i.e. it will glow in the dark). If you actually want a glow effect around the model, it will have to be custom done (usually valve is in charge of particle effects).

You'd be wasting your time. They haven't made any indication that the importer will allow for weapons. You should just focus on cosmetic items like hats and misc items.

Weapons aren't supported by the new importer, so if you want to make it for the workshop, you won't be able to. If you just want to make a mod, you'll need guistudiomdl. As for making curved swords, I can't really tell you. You just have to experiment and watch other videos on how to manipulate 3D mesh.