Ok, this chapter was just confusing. But other other plots aside, here is my possible theory for the ending:
Going back on my previous theory about the ex hooking back up with MC,
I feel that's still on the table as the flashbacks were vague enough to show that it wasn't a bad experience for her to turn him down if he did ask her again to be in a relationship.
But now I see 2 possible outcomes:
1) He ends up back with ex, after learning to take care of another person (Chris-chan/Knight girl)
or
2) Ex makes a move just as Chris-chan is heading back to her realm, but MC decides that its best to move on and stay with Chris-chan.
I feel it's favouring the 1st outcome because for the 2nd outcome, MC needs to decide to leave Japan with Chris-chan or Chris-chan needs to decide to leave her realm. Both of whom have not expressed any desire to do so. (Chris-chan maybe, but I don't think she has the option to escape back atm so staying in Japan is not a choice)
Probably down the line in this manga, both of them will be given an option to do so, but for me I feel it is a better writing standpoint to choose the ex over Chris-chan, not just because it tempers the "expectation", but realistically speaking, if you really love someone you wouldn't tie them down to your own circumstances (ex. Making Chris-chan live in Japan, when she has a family and duties elsewhere).
Letting each other go (MC + Chris-chan) seems like the best form of growth for the two of them. In terms of "living circumstances", I consider this like a transitional relationship, where two people who have traumas or lack of experiences get together for a sort of puppy-love phase, but when the two of them grow out of that, the choice is either stay with each other or move on and grow further. Ofc you'd want them to stay together, but if they do, maturity will not be possible because then they would be running away from their own responsibilities if they do (Chris-chan from her duties as a knight/princess & MC keeping her from doing so) in order to live this "fantasy" lifestyle they have created.
Whether or not this may be the outcome you want, for me this is the outcome which has the most potential for growth and has the best "message" which someone can take from in terms of their own relationships.
TLDR; MC will probably end with ex, because staying with Chris-chan will hinder their individual growth, because they're running from their own responsibilities by staying together.