Tbh i don't even know what Kanna was expecting, like she chose to come back here knowing Joe would still have Mitsuki to look after which is why they split in the first place and then i guess she thought she could get him to go with her? despite that things haven't changed for Joes circumstance. Like either way someone would have to leave their home in order to get together and its not like she said any of this to Joe and has only said something when she is planning to leave anyway.
I do believe its possible that they do get back together, whether that will be because of Joe deciding to go to America which he would probably only do if Mitsuki decided to leave with him or Kanna staying in japan which she clearly doesn't want to do.
I think Kanna wanted to test the saying, time heals all wounds. She might have believed that with time having passed between her and Joe, and now that Mitsuki is older, she could convince Mitsuki to move with her to the States. Unlike before, when Joe made the decision not to leave because of Mitsuki, Kanna likely thought that Mitsuki, being older and able to think for herself, might be more open to the idea. If Mitsuki agreed, it could have even influenced Joe to reconsider.