I think the developer wanted to give us a reason for a new start with another LI after leaving a bad relationship, so that he provides us with four very rich and satisfying alternatives. He left open the possibility of keeping the original relationship as a bonus, but never intended that would be a canon option, in some sorts. Most of the people would have enough reasons to pursue another life elsewhere after having their world views totally changed after a sudden revelation. Most of the plots I see elswhere are like that.
Sure, I don't disagree about that, the main reason why I think that it wasn't the original intention is purely because there is a choice there.
If the intention was to have them break up, doing so kinetically and then
later giving MC the choice after having her start out in the same 'position' as the other LI's would've been more logical if that was the case. And by position I mean that they'll all start out somewhat from scratch, since you'd have to build an entirely new relationship with Kaiju.
Of course, I don't have any evidence of anything, just a gut feeling from how everything was written.
Starting a game in a relationship can be great - especially for a story-focused game. Then you can go almost directly to lewd scenes without reverting to porn logic, and just focus on the story. Starting you off in a bad relationship is something different. For me there's no question of Kajia redeeming herself. I wasn't that into her from the beginning, and then there was the constantly being gone thing. When MC is offered a promotion at work and isn't sure whether to take it and then texts his girlfriend and gets no reply - I decided at that moment, in that playthrough (before the Templar stuff) that I needed to dump her if I got the chance so it's not like it was a good relationship from the beginning.
That's about where I also got pretty pissed off with her, more than I already was atleast. I mean shit, it never really felt like she was a girlfriend to begin with, the entire first part of the game I kept thinking "why is the vibe so fucking weird with that chick".
I don't think she had any interest in being a girlfriend to begin with, and as you said, I also don't believe that she was a bodyguard either.
Shit just reeks and I guess we'll have to wait and find out.