It's too complicated to create anything flexible enough to be generally useful, and QC isn't even used in Source 2

Never heard of it. The only reference I can find is a tiny mention on this page here.

If you find out more I can look into it, but it appears to only be supported in Counter-Strike: Global Offensive, which is soon to be replaced by Counter-Strike 2 on the Source 2 engine, so it may not be worth bothering with.

The Source Tools export animations for compiled game models, not Source Filmmaker. The formats have some overlap but clearly not enough.

The Source Tools don't do anything special with either of those UI panels. You should ask for help on a Blender forum.

Are you trying to import onto a hidden Armature? It should be unhidden automatically, but perhaps something has gone wrong there.

This happens with the VTA format. The file you are importing as just ancient and broken.

You could also have a bone scaled to 0. This can't be exported.

Looks like the file contain non-unicode text data. It should be utf-8.

This means that the model failed to compile. There might not be anything wrong with the export itself.

I meant a Blender add-on which reads and writes Source Filmmaker DMX files, sorry to be vague. Sorry also that I don't have the time or motivation to do this myself.