Some toxin exist that you can build up a higher tolerance than normal. One of them is Arsenic trioxide, it is well documented and the tolerance level of a "trained" person could be a multiple than the LD50 dose for normal people.
Don't think so.
If the person knew of MC poison tolerance and he want him dead, then he would use a much higher dose or another toxin that works for all the same.
If he wants to just scare them, then he would need the exact tolernace the MC have for this toxin. If he miscalculate and the dose is to high, then the MC could die. And if he miscalculate and the dose is to low, then the MC might not even notic it.
Therefore, I go with that the one who poisend the MC in the dinner scene wanted him dead, but didn't know that and used the "normal" deadly dose for him.
And with the same argumentation I don't think Rina wanted her son dead. She poisend him, but I do believe that the dose was to low to be really dangerous for him.
Perhaps it could be the only way she saw that her child could have a "safe" future. Risk her own life, paint a horrible picture of her as a mother, thus that the child would taken away from her and so end up in a safer place. Perhaps the contract with the Delucas and her son was on her request, so she could be sure that he is "safe".
The thing is, with MC's poisoning, I'm not entirely convinced it may have been with the intent to actually kill him.
Could possibly a ploy by either Wilfred or Gildart as a weird test as to part of "figuring out" the MC to see how he fares in the certain level of poison.
With Wilfred's (and Gildart, since he's Wilfred's currently favored student, given Cordia is Donna) resources, I'm guessing it wouldn't be too hard to analyze and find out the exact type of poison and the extent of it to find out how much of it would be good to test the MC.
Of course, something like this Cordia would never condone, but the game does show Wilfred doing several things behind Cordia's back (some things she turns blind eye to them, some things she calls him out, but I'm also assuming there are some things which Wilfred does that Cordia doesn't notice at all).
As for Rina and her intentions?
While I guess on one hand it could be she had good intentions in mind, on the other hand, MC states to Luna and Gracie (in the Beach Prison event) that she started becoming less motherly when his father left them.
I dunno, I think if they wanted the MC to have a "safe" future, there could have been alternative ways to do that even when his father was around, and I do find the whole poisoning to make herself seem unfit kind jumping through the unnecessary hoops (when in fact, if they wanted to portray themselves as unfit parents, they could have just abandoned the child MC at an orphanage without the whole poison endeavor).
Perhaps it's a bit of cynic in me, but I'm getting a bit of feeling that Rina just didn't take MC's father leaving too well, and that emotional stress (and possibly even breakdown) seeped out to how she treated the MC.
And I do admit I might even actually prefer it that way.
For conflict issues that MC has encountered within the game that he had (or has) to solve, a lot of them stemmed from either "misunderstandings" or "either parties not getting the full picture." such as Gracie & Isabel's reconciliation, and likely Luna & Cordia's future reconciliation as well.
For Rina & MC, I guess I'm keen to see something........ different.
Rather than Rina saying to MC "I did all these actions for your own good." and MC coming to terms with it with the whole justification, I do think it might be a more intriguing narrative if she has a real heart to heart talk with her son realizing how she didn't handle the situation well (with her mistreatment of him due to stress/breakdown).
Hence no misunderstanding or the not knowing the full picture twists and turns or secrets. Simply being the case of bad parenting.
Would be a monkey wrench thrown with players expecting characters actually related to MC being either good (or decent), and would also show that not every issue can be fixed with same fix.
Sometimes, it's good to have characters admit to others that they themselves have a problem and face it head on.
Just my two cents on it, though I guess I could be totally wrong (and yes, I have been wrong before in my theories also, heh).