You know the game way better than I do, but a murder suicide using poison, is very unrealistic as far as the real world goes, basically unheard of.
They usually use a method that instantly kills, and someone like Rina ? It would've been pop-pop and over with, if she already made the decision to take her own life as well.
And do you really believe, that there is even half a percent of a chance, that she wouldn't have managed to kill herself by now ?
I don't think the Delucas or anyone could've stopped a murder suicide, if that person is commited. Furthermore, since none of the Deluca's even knew that the MC was being made to ingest poison by Rina probably means that you overestimate their actual surveilance on them.
I mean, there have been times (couple of times, actually) where I was wrong in my theory before so how much I know the game is not really that much of a factor in my opinion.
In the end, it all comes down to Hopes, and what he has decided already for Rina's character; while there are certain elements which get changed/altered down the line like character preferences (for e.g. Onorina initially was supposed to be a much smaller character, yet her character's skyrocketing popularity got her more scenes in season 1).
Hence I wouldn't dismiss that possibility that Hopes possibly set it up as something intentional.
Having said that, I think it's a mistake to dismiss the alternative (i.e. she really did mean to kill herself) basing realism as reasoning.
Basically, the whole game leaps the bounds out of the realism with the following; characters, especially high ranks perform prettt much superhuman feats of dodging bullets, bullets changing trajectory, rollerskate dance of death and so much more.
The entire DeLuca universe is filled with exaggeration; and that is part of its charm.
So for me personally I wouldn't use the "that is unlikely because it's not realistic." as reasoning.
Heck, even if I humor for a moment the whole realism reasoning, it's not like there weren't any suicide by poison in history in real world. But that's a whole new can of worms that I won't get into.
Well, none of us know what Rina was going through at that point in the flashback.
While on one hand you could be correct in saying "she could have had all this planned." it can also be correct in saying she just wanted to end her and her son's life due to her dispair.
And yes, she could have just ended her and her son's life by slitting child MC's throat and same to herself. But who knows? Perhaps she wanted to let her son die in a less gruesome way without the blood.
So MC getting the tolerance for the poison could very well have been unintended consequence of her failed suicide attempt.
As for whether I believe she wouldn't have managed to kill herself by now, well....... I believe she could have very much tried, multiple times. However, we must remember Rina after the poison attempt would have been very much weakened; so Cordia and Wilfred could have left 2 or more high ranks (or ones just as capable) to act guards in order to prevent her from killing herself (kinda like orderlies from psyche wards physically restrains and using sedatives to patients who tries to hurt themselves and others).
While Rina at her prime would have handled such guards no problem, her poisoned state it could very much be a different story - and that could very well add to Cordia's guilt storyline, pain at having to see her men restrain her best friend (so not just with Luna and her husband Lucan, but also with Rina when it's about Cordia's guilt).
But in the end, it's all gonna come down to what Hopes decides.
Hence, until that time I'm being open to both scenarios, even though I lean just slightly towards the "she tried to kill herself and MC" one.