First things first, JQ's translator note intro panels always crack me up, the hamster collecting the garbage was SUPER CUTE and when the coworker said "the person you marry should be someone who earnestly does their share of the housework" I thought "hmm... I mean, sure, but I think there's something that's more important than that isn't there? Like a little something that's ends with 've' and starts with 'lo'? No?" lol
But as for the big topic, instead of preemptively freaking out I'm gonna point out a BIG silver lining here and a reason why everybody should be excited about rule 5 existing. Sure, it points to some drama in the future (though I doubt it's gonna be too dramatic, I bet it's just gonna be one of them thinking the other is in love with a third person when that's not even the case), HOWEVER, it also means that this manga won't just be "these two girls are married, but they're basically roommates and there's not much development", it means love will eventually play a big part in the plot and, ofc, that they'll eventually fall in love with each other after the expected little drama.
Basically, rule 5 existing confirms that there's gonna be real development in the future, which is great.