It's a bug in that version of Blender. Upgrade it and the Source Tools.

Yes.

You can use Blender's built-in text editor. The Tools will generate a Text when you click on the button mentioned in the documentation.

You need to use advanced flex mode and edit your flex controller DMX element.

https://developer.valvesoftware.com/wiki/Blender_Source_Tools_Help#Advanced_mode

Each animation was imported as an Action, but Blender will only display one action at a time.

I added a small UI element to the Tools interface to help you switch between actions.

The path isn't a problem. The file has characters that aren't valid in Unicode 8-bit. You'll need to fix that somehow.

Looks a lot like you don't have the tools installed, or that they are disabled.

That's from HLMV by the look of it. Studiomdl will remove unused bones from the compiled output, so for them to appear in that list you must have an animation which is moving the objects directly. Presumably that is causing them to be promoted to bones at compile time.

Meshes are not exported as bones ("joints" in DMX terminology). What are you seeing in your compiled model?

If your decompiler writes SMD then you are stick with SMD, yes.