Source will ignore bones which don't have any attached geometry. You'll need to create an Armature modifier for your mesh.

This looks like an internal Blender error. Try reinstalling, and if that doesn't help ask on the Blender website.

Create a parent item for all of your objects, then they will all be exported relative to that location. The parent can be anything including an Empty object.

Blender 2.79 hasn't been supported since 2019. You need to update.

There are many "drive" websites that let you upload files and share them. Any of these will do.

Please upload your blend file and screenshots of the problem.

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.