Zouken did other things besides the worm sessions to make her body more resilient, he even resorted to poisoning. Rin went down into the basement and saw at a glance what the training would be all about, and even admitted she wouldn't be able to handle it, as she said that only a small amount of knowledge is repaid with the torture, and the subject is likely to go mad before learning anything. Why do you think Kariya left and decided to never come back? The training is humiliating, inhumane, and degrading with pretty much no benefit. It might have been different when Zouken was trained by his family, but that is not the case here. Zouken pretty much admits that his family are pawns to be used, and the Kariya and Sakura attest to the heir being nothing but a facade, a thrall for Zouken to use to capture the Grail.
As for Tokiomi, he made a mistake due to blind faith. His part in the story, along with Kayneth, is to show the folly of the ways of mature magi, and their hubris. He made a pact of Non-Interference with the Matou, not even knowing much if anything about them, and that was it. There was nothing in the story to say he ever really saw Sakura after that fact. This is dangerous, because not knowing what magecraft they use, or what effects it had. This means not only was he clueless, but he pretty much took them at their word and left a four year old with people he didn't even know. No supervision, no letters, no visits, no calls.
A child's magical talents are linked to the blood of his family, and their is no way for other children to use the magic of another family without changing their body, and to a fairly large extent, ruining the talent they were born with. As we have seen in each story, there is pretty much no exception to this. Sakura is the only example we have of a child born from another family and having their body acclimated to a new family's magic. The fact that there is still such large potential available to her(once she is free of course, the worms acting in their parasitic relationship take as much energy as they can without killing her, which would make it pretty much impossible to use magic), and the fact she is still alive after all that is because of her strong bloodline. Ironically though, Tokiomi, even for all her wanted for her, had stolen her talent and forced her to have to work thrice as hard from the day she was born by not training her as a Tohsaka.