Answers to your questions:
This is a confirmed bug, will be fixed for next release.
Tried to replicate it but no luck, could be unsupported alembic format. Could you share the file?
Also a confirmed bug, will be fixed for next release.
Yes, the label should be "Houdini nodes" not "Houdini binary", dictating if selected/all nodes also should be exported to a .hip. "Attach scene to asset" will generate a new scene containing selected/all nodes.
Import creates a new subnet node and import all the nodes beneath, while merge imports the