Your memory is excellent. Yes, they had to argue for two transfusions, because of the timing of the CB and DEHP positives. That did complicate their case, and made it appear a stretch, though I always felt it was quite possible, even what a rider would likely do (the plasma being used to lower HT following the red cells).
However, the part about another blood (plasma) donor. This was put forth because the prosecution bought (or feared) the defense’s argument that Contador would not have withdrawn blood right after a large dose of CB, which had to be the case for enough CB to appear in a transfusion to trigger a positive. But this is debatable; Contador would not have known at that time that such a sensitive CB test existed.
In the end, I think the decision was intended to satisfy two criteria: 1) it couldn’t be meat, and 2) the transfusion scenario was too complicated. Ruling that it was a contaminated supplement was one of those worst of all possible decisions except the alternative. The irony is that if Bert had argued supplement in the beginning, he might have gotten a reduced sentence. But au contraire, he insisted he didn’t take any supplements.
It's interesting to speculate what would have happened had he been given a one year suspension for the supplement. If the sentence had been handed down by the Spanish ADA, in early 2011, he would have missed that year and never won the Giro. He could have ridden the TDF the following year, but based on his performance in the Vuelta that year, barely winning it, I doubt very much that he would have beaten Wiggins or Froome in the TDF. So ironically, getting two years probably helped his palmares, by forcing him to ride a GT he could win.