This pardon is a signal to those that Mueller would be hoping to flip that Trump has their backs if they don't squeal. There is no other rational explanation for him to intervene in the middle of an ongoing legal process.
Vile on every level.
If Trump pardons those people, they can still be called in for testimony where they have every reason to no longer need to feel they have to lie.
In fact, it's in their interest not to lie since they could be convicted of a crime again where as far as a second pardon goes:
Not sure if this applies federally.Two times. Typically, the same person can't be pardoned twice unless a majority of the state supreme court approves.