I agree with you that there's no point in blocking Mental if they've already checked him. But blocking someone else, whom they considered a "kraken" for example, is a perfectly valid move. However, they ended up hitting Jack, who then targeted Mental. My theory is that this is exactly what happened.
This is possible, but it seems unlikely. It would have been easier to simply kill Kraken with Beckett, since the chance that Kraken would hit Beckett is minimal.
Если где-то чего-то убудет, то в другом месте добавится.
(Закон сохранения).