I'm far less sure it's so obvious. My initial feeling was she's simply tricking them for some purpose. She tried to get Crona to find the hidden demon tools in Shibusen, so she's interested in something over there. I don't see, though, her wanting to destroy Shibusen right now. She's clearly not going to cooperate happily with Arache so she needs far more desperate or perhaps creative measures, especially since she doesn't seem to have too many underlings.

But I don't see her trying to form an alliance with Shibusen, nor would I expect the folks at the Shibusen to be too eager to do that either. Shinigami is strange so in that sense all the options would be open, but Medusa is the one who freed Kishin in the first place creating all their problems so it might be too strange even for Shinigami himself.