I think that Beni's CHF and Zen's heart condition wouldn't effect them the same way. With the CHF being an ongoing disease that was probably caused by a heart condition of some sort, think of Beni as far more advanced in illness than Zen. He's been dealing with this problem for quite some time, so the long term effects may have caused the white teeth. As already stated, genetic predisposition could also play a role in it. This also applies to any other illness. Their teeth don't turn white from a week long URI. It would be something very long term that would cause their bodies to something like that. Genetic predisposition could lay a role.
As for lack of nutrition/inability to absorb nutrition, this would be something long term. It could have been from the very beginning, when they were first born that the mother wasn't receiving adequate nutrition, so the bubs didn't, either. It could have been from a prior owner who consistently did not provide adequate nutrition. These are all long term things, not something that would happen after a couple of days or weeks with a bad diet. Genetic predisposition also would play a role in this. If Rat A comes from fairly healthy and hardy lines, then she won't show as many symptoms of being fed a bad diet. If Rat B comes from lines that are predisposed to illnesses, it may be more likely to show up there.
Twitch, as far as cavities go, I've never come across any articles or case studies regarding tooth decay in rats. As far as I understand, cavities aren't something that are 'naturally' occurring in rats. Maybe if they were fed a ridiculously high sugar and/or acidic diet it would wear away their tooth enamel to some point, but I'm not sure to the extent of dental problems such as that.