If I follow these steps and then export selected objects or the scene, I get the mesh and the animation.

The best thing to do is just not put the armature into the collection, or if the collection isn't supposed to represent a single export, suppress it from the Exportables panel.

However if the scene isn't configured that way, then I accept that it's confusing that the animation isn't available for separate export. Perhaps armatures with animations should always be considered as exportables in their own right.

You need to match the up axis of the model you are animating. If you aren't sure what that is, just experiment.

I don't know about the keyframe issue, this is likely to be something that you can configure in SFM.

If the engine branch isn't recognised, you can still specify the DMX version and format manually. If you are exporting SMD then you don't need the engine version at all.

This is probably because you scaled the mesh/armature without also scaling the animation.

You need to use at least Blender 2.92.

Please upload your blend file.

This error means that your file contains text with invalid characters. Try ensuring that all names in the SMD file contain only Latin/English characters.

Never seen this before, but it looks like a Blender error that multiple add-ons are affected by: https://projects.blender.org/blender/blender/issues/99440

Sorry, got confused. It's a problem with the SMD file. Where did you get it from? Can you upload it somewhere?

That's an issue with the QC file, not the SMD file(s). Try importing the SMDs directly.