but and you seem to think he has carte blanche to pursue suspicions of crimes that are completely unrelated to the central purpose of the investigation
He does. Surprised? These have to bear fruit, though, or be tied off and the investigation brought to a close. If these other crimes aren't successfully used as leverage for information, while they can still be prosecuted, Mueller will be out of bullets and the investigation will be de facto over (in lieu of other evidence or leads).
Fishing expedition? Ha..... if only my "fishing expeditions" were so successful! I would have to take an extra boat to carry back all those fish!