I think MM knew she could never stay at Buck Palace after saying that she wanted to kill herself living while under the Establishment thumb – why stay somewhere that might trigger old trauma? Sussex Squad would find it too baffling.
However, they still need to be seen as Royal for the $$$$. So, Harry staying alone at Buck Palace means he gets the security protection he so deeply craves and reminds everyone he's still a Prince.
BUT, yeah, the optics look really bad for MM now. If she won't even come to Buck Palace, where they'll get armed security, then it shows she's got no intention of helping him reconcile with his family. Which I think will ultimately drive them apart.