It is clear Mueller knew there was no collusion a year ago because his indictments stopped about then. Mueller engaged (continued) an investigation that may have been reasonably premised on apparently fraudulent evidence, he found the evidence flawed and the justification for the investigation into Trump's collusion with Russians to defeat Hillary without merit. As it became apparent there was no big there there, Mueller resorted to these indictments, none of which have anything to do with collusion, but do relate to other wrongful conduct.
About a year ago all Mueller had was the possibility of coercing some admissions from people associated with Trump for wrongful conduct of their own in whatever dealings they made that had nothing to do with collusion. For example, Mueller discovered Cohen was not properly reporting the foreign citizenship of NYC taxicab medallion buyers, this is a process crime, but it is indictable, Cohen was charged, he admitted the wrongdoing and agreed to cooperate in Mueller's investigation of collusion. Since there really wasn't any collusion, Cohen couldn't provide any evidence for Mueller, in the end Cohen cops a plea, yes he's been indicted, this was Trump's lawyer, Trump looks bad, but it is clear there's just a faint hope someone on Trump's team, caught in some improper deal will confess to this collusion.
The cases against Manafort and Page are analogous to Cohen's; Mueller found evidence they did lobby without being properly registered, its another process crime, they get indicted, are persuaded to cooperate, but again, they've no evidence of collusion, they can't provide Mueller with anything, cop their pleas and Mueller needs to see what he can get from Papadopoulos or someone else.
This pattern suggests Mueller went at it, came up empty on collusion, uncovered sundry entirely unrelated misdeeds by Trump associates, leaned on them to extract collusion confessions, got nothing and finally had to give up.
The notion Barr, Mueller, McGahn or anyone else can offer testimony that will reveal Trump's collusion with the Russians is absolutely ridiculous.
What I do think could become evident from their testimony is a strong suspicion Mueller kept the pointless investigation going for a year after he knew there was no big there there, just in order to promote circumstances when Trump's conduct could be described as obstructive.