That still doesn't make any sense, because it would mean the traitor is part of the kitchen staff, which means really easy to track down and on top he or she has to be rich enough to hire an assassin from the ombra family and like 50 mercenaries.
As shown by the incidents with the mercs and the Carnefice lady used as a cat's-paw by the Ombras, though we don't know
how big exactly, whoever wants the MC dead is definitely "crime family"-level rich.
They
already got a confidence man of the capo to rat out as shown by the beachhouse incident and the car sabotage demonstrates some degree of forewarning about the MC's coming and goings. So subverting a member of the kitchen staff is within the realm of possibility. The DeLuca's security structure is extensive but is far from unassailable, as shown repeatedly by several examples through the story.
And again it doesn't explain why it was exactly the one poison that Rina used.
It's not like Cordia will announce who will eat with them by name, she will pick up her phone tell them "1 more" and be done with it.
AND how many
unexpected visits in the same day would they have for dinner?
Almost anyone else invited to the DeLucas table would be someone either notorious in the crime world or close to the family, ergo, produce gossip by their mere presence - so the absence of "noise" on itself made the chances of "the extra" being the infiltrator's target, aka the MC, high. Maybe a gamble, but the same could be said about the sabotaged car brakes, no?
And about the poison - popular tools in the field. Like with guns, some brands/types of substances will be more popular than others due to a combination of availability, potence, perceived effectiveness and so on. MC's mom was very much a part of this world, so her picking of a poison that is actually an "old standby" among assassins?
And resistance aside, he
would have died at that dinner if the family did not rush in a (possibly in-house) doctor. That Gracie knows what mithridatism is doesn't mean many people in the underworld do, on the contrary if Antonio serves as indication.
To be honest, the MC's original poisoning survival back in childhood is the insanely improbable thing when one thinks about it, but then everything in comments and flashbacks seem to indicate his mom was a pale shadow of her own self and quite off her rocker by the time she pulled that stunt.