You mostly answered your own questions.
Although, Bella didn't really lie to MC, she "omited" the truth more like, if you play Nora's path she actually answers the question
So technically it's okay for MC and Bella to date, it's just that according to the rulebook they'd have to make their relationship public and apparently she isn't ready for that yet, but as you said, she realized she needs MC so she doesn't want to end their relationship either and decides to hide it instead.
My best guess is that she's afraid MC would get upset that there's a "legal" way for them to date but Bella chooses the "illegal" way instead. But for MC in EP8, legal or not, for him it was just a matter of hiding the relationship, I'm sure he'd understand Bella and respect her wish if she told him the truth, so I don't really understand why DPC wrote this plotline this way tbh.
Moving on, what did you expect her to say about James? In earlier episodes she clearly still has some hope James might come back one day, she probably still loves him, do you really expect her to give up on her old life that she's been hopelessly hanging on for 3 years, as soon as she meets a man whom she grows feelings for?
Put things into perspective, by the time she and MC have that talk they have met around 3 months ago and started truly dating (not fooling around) about 1 month ago. Her marriage with James was probably a decade old or more by the time he left.
All she knows is that MC gave her hope and a resemblance of happiness for the first time in 3 years, for now that seems enough.
All in all I think we should just be happy that she finally opened up a bit and was honest about James with MC, that by itself is a huge step forward and shows how much she cares about MC already.
As for the dinner, your guess is as good as mine.
And it's not all doom and gloom for her branch, c'mon, it's quite romantic as well, did you play the same episode as me? Their date is super funny, almost every line that woman said can be turned into a meme. And their relationship looks wholesome and natural, something I thought was going to be quite hard to pull off considering the woman we saw in earlier episodes.
Oh, and the details, pay attention to the details, they're adorable