Unpopular comment but not really sure im on board with how Mahoro apparently having no problem lying straight to people's faces, all while being hypocritical at the exact same time, when all she has are hunches and guesses.
Now I completely agree with the majority of what you're saying here, but firstly let's remember this Josei. He be cheating, every man who might be evil is definitely evil (unless he's the main love interest then its all just him being a misunderstood badboy).
So lets get that out of the way: She's doing questionable behavior, her reasoning is VERY shakey, but she is going to be borne out to 100% correct because its manga for older women.
Now that said:
Mahoro isn't completely out of pocket here. Even if Rei wasn't cheating with her best friend, he is a shitty partner (a comically one-dimensionally shitty one, but again: Josei) and Mahoro is done with the relationship. That's perfectly in her right if she just decides things are done. She's not been cheating, she hasn't tried to line up her next relationship, she hasn't let herself be seduced to some idealized life. She's just done being his fuckmaid-but-without-the-fucking and that's completely her perogative.
(I agree with you that Rei hasn't shown himself to be anything other than inconsiderate and controlled by his mother, these are fixable issues, but Rei isn't showing any signs of self improvement and its not Mahoro's job to fix him. But also keep in mind: he's definitely cheating on her, that's how this will turn out.)
I think what makes her actions "OK" is that she's not
harming Rei, she's
helping herself. She is looking at newly single life and realizing she'll both need money saved and will need to have her career path elevated from just "entry level grunt work". She is very clearly intending to save her own money, not take Rei's or the house hold's savings to do this. So she's being deceptive about her reasons but its not like Rei isn't also being deceptive about cheating.
The only thing Rei is really losing on is Mahoro's bomb-ass curry (and that's questionable, she'll probably be expected to make dinner most nights still) and he could learn to cook himself.
The other thing to consider is "what if she's wrong"? If this wasn't just female wish fulfillment manga, Rei is completely innocent of all NTR charges, and Mahoro has a change of heart about not saving their marriage:
Very little harm has been done. She worked more for a couple years, Rei didn't have some curry, they have some more money. It would be very easy to walk back her decision.
Also, Rei says he will cover the extra costs right? Don't you think if a husband was cheating with you. That he would WANT you to be full time? Since then you're even more busy then before and then he would have more available time to cheating?
Did want to address this part more fully:
Plenty of cheating partners have no intention of leaving their "primary" or otherwise upending their normal life. Rei is very likely intending to have his curry and eat it too: have a well-kept home with dinner waiting, and bone Rio on the side until they tire of thrill. He just doesn't want to lose his housekeeper.
The other side is if Rei IS caught, if he makes all the money he has all the power. He could just admit the affair at this point and Mahoro wouldn't have much choice other than to accept it. That's likely working into his mental calculus as well.
There's also a japanese cultural thing about "being superior" to people/looking down on them but I'm going to get into that.
I think there have been jdrama/manga where the husband is getting the 'allowance' despite being an office worker since the wife balances the checkbooks at home or so for groceries/errands(?),
This is actually pretty common for a lot of couples I know. Both pay into a joint account and both get "allowances" to do with whatever they want without the other person saying boo. If there is a "couple cost" (trips, car, saving for home/home improvements) they both pay into a fund for that.
One couple I know with income disparity, they split household expenses like rent and groceries on a ratio of income; e.g. he made twice what she did, so he paid 2/3 rent she paid 1/3.
well she just mentioned eggs but not buying any 'baby' stuff but i imagine
she can't just ask for like a huge amount each month without it being suspicious (and if they do go to some divorce court he could prolly countersue to 'get it back' versus her owning her own pay)
This right here. Mahoro knows if she touches "couple money" before the divorce, it'll erode her case and reduce Rei's "fault". By making/using her own money and presenting evidence of infidelity, she will be the 100% aggrieved party.
She doesn't 'need' permission but i imagine it'd cause issues if they have a divorce hearing or just letting him know so he doesn't complain about her 'slacking' (i bet he could just go out to eat with his affair partner anyways unless they literally only meet at hotels [but wouldn't be surprised if love hotels have room service too, did see some that were like 'female friends' service, not necessarily gl/yuri but just as a 'girls night' thing with champagne])
Its sort of fucked up, but it is not completley out of pocket for spouses to call the other person's place of work and berate their boss. I wouldn't put
Rei Rei's mother above calling Mahoro's place of work and making her boss cut her back to part time to make the crazy woman go away.
and Short-stay hotels above fleabag levels most DEFINITELY have room service and its usually very good.
How rich is he to shoulder those costs? I can only guess that his earnings are way more than the household necessities + rent/mortgage + the planned child + his expenses + her expenses and he still reluctant for her to work?
He is likely making a good bit even a junior salraiman. enough to absorb some extra food costs no issues to be sure. Prices are ... different... in Japan. Most day to day purchases are cheap, but things like clothes, cars, and houses are crazy expensive. OTOH there is no culture of massive rent increases, so generally when you rent an apartment that's what you'll pay until you move out.
My question would be more to the tune of "why is he needing to cover the costs"? Like no joint account?
Why the fark is she even asking for his permission?!
Because that's how its done in Japan. Remember, consensus-based culture. Couples are supposed to consult on these matters. If he wanted to quit his job, that would need her permissions. This is also why "no no-fault divorce"; you are not to make individual decisions.
The reason she cares about going through the motions is going from part-time to full-time work is not going to be something she can keep a secret. Rather than a blow up where she might reveal her hand or make Rei suspicious, she's playing the long game.
the cheating at this point is completely irrelevant my dude, he's been talking about having a baby since chapter 1 while not having sex with her, he is completely uncaring and oblivious as to how this might appear to her, the most likely scenario is that he's been wanting out of the relationship for a while and he doesn't wanna get tied down to her
Yup. Lets not forget what started her down this road of discovery was the fact Rei wasn't fucking her. (and also while the cheating part is irrelevant, remember it is going to come out he's cheating. Josei gonna Josei.)
Also, of course Rei doesn't want to be tied down. He can't elope with Haruto if he's got a kid. /s