I suggest sticking to Blender 4.3 for now.

I'm telling you to use Blender to edit your scene. Actions are no longer exported, just slots.

Luckily for you I have found how to do it. First you will need to make multiple copies of your armature and assign one action to each. Then you make the "real" armature the active object and execute the "Merge Animation" operator in the Dope Sheet / Action Editor:

https://docs.blender.org/manual/en/4.4/editors/dope_sheet/modes/action.html#action-menu

Afterwards the active armature will have one action slot for each other selected armature, and you can delete the armature copies.

Is this something you see when loading a dmx file? Try exporting to an earlier version.

You need to convert your separate actions to slots on one single action assigned to your armature. Then you will get the same behaviour as before.

I'm not actually sure how to do that, but there must be a way...right?

I can export the data from your file without any error. That said, I can see why the exception is being raised.

I created a branch which will fix that particular error. Please try it out and tell me if you encounter any other issues:

https://github.com/Artfunkel/BlenderSourceTools/tree/scientist_mdl

The importer doesn't do anything with sharp edges. Any sharp edges you see are something Blender did by itself.

There is no need to guess. It will export whatever you are seeing in the Blender viewport.

I've no idea, you should ask this question in a general Blender support forum.

The exporter will write the actual loop normals of your mesh as calculated by Blender. Edge sharpness will alter these values. But edges themselves are neither exported nor imported.

Looks like corrupt geometry to me. Your shape has fewer vertices than the mesh.