Did you know that Andrew Weissmann was briefed on the "dossier" in the summer of 2016? That's an easy one to start with. ... If an attorney is involved in the chain of custody of the evidence he shouldn't be involved in the investigation itself. I'm not a lawyer myself but all the lawyers around me ... say that's like in the 101 handbook of what not to do as a prosecutor.
There are plenty of other simple, binary type questions of that sort. Now that Judge Dabney Friedrich has slammed half of the "collusion" narrative, I strongly recommend--as have various commenters here--that the GOP members focus on the other half: the claim of Wikileaks and Russian involvement in the DNC "hack." Again, just simple questions regarding the failure to take custody of the physical evidence and yet conduct investigations and prosecutions that relied on third party claims (by Crowdstrike) about that evidence.
The whole half hour interview is fairly interesting.