I don't really sculpt, but I can answer several of your questions:

The high poly model is only for baking light maps, normal maps, etc. The actual model used in-game is the low poly version which usually have tri limits of 200-1000 or something. The high poly sculpts that people make have hundreds of thousands (if not, millions) of tris so these are definitely not viable for use in-game where real-time rendering takes place.

If I were you, I'd pick up sculpting if you want to do dota modeling. It's more necessary for Dota whereas in TF2 you can get away with low poly modeling the entire way through. I'm not really a professional or anything, so I can't really recommend what type of program to use for sculpting, but I do know of three good ones: Mudbox, z-Brush, and 3D Coat.

And the rendering of the maps can be done externally or within those programs. I think Lenny/Vlad might have some tutorials regarding the whole map-baking process.

This is just too cool!

I don't really know if they handle it any differently. I have a pretty successful workshop friend who is under 18. I'm guessing your parents will have to deal with the monetary side of things until you can do it yourself. No shame in that, though.

Looks great, but I would size up the blade by a lot!

I actually don't know how to do it in Dota 2. I would tell you to use a $attachment line in your QC file, but Dota doesn't deal with QCs. Either Valve does the particle effects for you, or there is another way to do it. But I don't really know, so I'm sorry to say I can't help here!

Diffuse is essentially your main texture.

There are tons of textures you can have:
Diffuse = main texture
Bump/Normal = shadows
Light maps = lighting
etc.

http://forums.steampowered.com/forums/showpost.php?p=34536858&postcount=2460

Animation frames by defautl are set to Bezier interpolation, so you'll need to make them linear if you want linear motion. You can do this by selecting the appropriate keyframes and pressing "T" and then selecting "Linear" as shown here: https://dl.dropboxusercontent.com/u/10100459/BlenderImages/linearAnimation.jpg

Oh. Yeah so Blender has a feature that allows for transparency depending on your alpha channel. I can't remember exactly where that option is (or how you turned it on), but let's see if a quick google search shows up with anything: http://wiki.blender.org/index.php/Doc:2.6/Manual/Materials/Properties/Transparency

Maybe that?

Hm... you might be messing with the $additive or $translucent commands in the vmt. Try turning those off.