Thank you for investigating! I've fixed both issues now. Looks like __getitem__ was added to the Python string type since I wrote that code! The other issue is a cheeky hack that I wrote a very long time ago and then broke earlier this year. I'm not going to refactor that into something good right now, I'll just patch it up.

Something went very wrong when the script was being loaded. Restarting Blender would probably fix it.

I've no idea there, sorry.

Not without either drastically slowing down the import process, or producing vertex mismatches in dense meshes.

Use DMX if you at all can, it's a much better format.

Wow, thanks for spotting this! I've corrected it now.

My guess is that the file was exported as ASCII with some non-English code page that collides with UTF-8.

This is normally an issue with the material not having the correct path to the texture. It's not related to the exporter.

Looks like that version of Source doesn't allow vertex animations to move a vertex by more than 8 units.

Vertex animation was added for Dota 2, so it's not very surprising that older games don't support it properly.

It's the new PointerProperty which changes behaviour. It adds a user to each object in the scene export list and thus causes Blender to not delete them from the Blend file when they are removed from the scene/view layer. (I was evidently wrong when I said that deleted objects remain in the Blend file until it is closed.)

Immediately after the object is removed the scene's export list is updated, the PointerProperty is removed, and the user count is decremented. But by this point Blender has already decided not to delete the object fully.

The extra user is annoying, but I'm not in a rush to change this. Even if I did change it, you still cannot assume that deleting an object from the scene will remove it from the Blend file: the object could have additional users for many reasons, e.g. from modifiers, other scenes, or a fake user. You should keep the changes you made to your script.

I don't know which DMX formats Garry's Mod wants, so I can't include it in the add-on. If anyone does know, I'll add it as a recognised engine branch.