I think you’re right that focusing too much on Hillary lets Obama off the hook. But the source of the dossier is important to the extent that the dossier was the source of the information that was used to obtain the FISA warrant. It’s one thing if the FBI on its own came up with enough intelligence to obtain a warrant; it’s quite another if they had to rely on paid-for opposition research. Did they disclose that to the court? Keep in mind, they were denied a warrant in June. Then they get the dossier, then they get the warrant.
In other words, the fact that Hillary paid for the dossier does not let Obama off the hook; it actually makes the Administration that much more culpable for using it to get a warrant.
Getting Hillary’s campaign involved also could have been a form of insurance. If she had been elected, she would have been too compromised to ever allow information about illegal surveillance to become public. (In fact, she most likely would have been compromised for lots of different reasons.)
This entire story is starting to have the earmarks a very sophisticated intelligence operation, with information being laundered through multiple sources. Who in the US government is most skilled at this kind of work? It is there that I would start looking for the originators of the scheme.