Hi Yating,
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 nodes into the current scene with risk of possible node name conflicts. We can't replicate the issue you have with Import generating an empty node, could you share the .hip?
Cheers
/Henrik