I also genuinely think it’s not that deep and that there’s no complex mistake being engineered here. I don’t believe you that the “mistake” of “sa with a replaced by i must be si” is an an unusual one for someone who hasn’t yet internalized kana. If we test this on random people on the street, I’m highly confident an overwhelming majority will make this exact mistake.
I agree with your broader point that “teaching via mistakes” is a risky path not worth it when the mistakes start getting combinatorial. I also think it’s absolutely fine when everyone does the same exact mistake, and there’s exactly one way to avoid it.