Just quoting this bit for brevity's sake.
Your rant kinda confuses me. At what point was it said that any of these girls were gonna "cheat" on the Manga-MC? Granted, these girls are meant to go after the Game-MC...
....But that does not make them cheaters if they have no knowledge of this themselves.
Look at the film, Back to the Future, for example. MC Marty McFly goes back in time and accidentalies the first meeting between his parents. As a result, his then-teen-aged mom winds up going after Marty for a while - she has no idea whatsoever that he's actually her son from the future. Granted, things did get pulled back on track, but up until his past-dad clobbers someone, there was absolutely nothing going on between his past-parents. And none of that made his past-mom any sort of cheater. Eager to get her game on, yes, but nowhere near any sort of cheating because that requires her to be 1) in a relationship already and 2) fully knowing of what she's doing. Neither condition applied at all.
And that's pretty much the same here. The only girl who could even be capable of cheating at this point in the game's story is the childhood friend. The Saint and the Swordswoman hardly even know the Game-MC as anything more than a classmate.
It's literally written into their personality to cheat already bruv... cause that's what they did cannonically.
Once a cheater always a cheater, cheaters don't stop cheating magically for a special guy. If given reason to they'd do it at the drop of a hat.
Which was my point, it's within their personalities, making them shit LIs for any character you care about, especially the MC.
There just has to be a dude that comes along that they like better and boom they'd cheat.
That Back to the future example, has no relevance, because the Mom never was a cheater. Where as this example, all these characters are going to cheat, you know their future, you know what they are capable of doing.
So why would you want an LI that is capable and would easily cheat, over an LI that would rather bite their tongue and die? (the maid seems to be one such LI btw).
MC knows all this as well, so he'd be a fucking moron to take any of them seriously, because that's what they'd do to him as well, because they did it to the OG MC. And unless the story actually goes "Btw that game was not actually canon, and they'd never do that" (which it doesn't) then they're cheaters waiting for the opportunity to cheat with someone they like better than MC.
I.e. they never took off the "For sale" sign, and are just waiting for something they like better to come along.
Lets make it even simpler for a second, since "Cheating" aspect might be confusing you. Lets change it to "betrayal" if you know someone is a traitor, would you trust them? would you expect them not to betray you, when it's literally part of their identity to betray? No you wouldn't you'd be a complete moron for doing so.
Well this is that, just the form of betrayal is cheating.