The "Verification" requirement is to assure the FISC that the application is authorized and made by the FBI and DOJ, and it is not the effort of a couple of low-level employees operating on their own in rogue fashion.
There were 1300 FISA applications filed in 2017. https://twitter.com/Quirk22/status/1293991003792252929

If you think the "Verification" requirement is that the FBI Dir. has personal knowledge of each case, and "verifies" the factual accuracy of everything in each application, that would mean he would need to do that for 3.5 applications a day, seven days a week, 52 weeks a year.

There is no intention in the process that every senior manager whose signature is attached is warranting the factual accuracy of everything stated. The only person that applies to is the "Affiant" who sets forth the facts of the investigation in support of the application.

But even that signature is based the premise that the facts set forth are "true to the best of my knowledge and belief." PURPOSELY including false facts to influence the FISC's court's decision is likely a prosecutable offense. That is where the line is drawn.

Who knew of and approved of purposely including false factual information with the intention of misleading the FISC. That is what Durham is searching out.
He has to PROVE it beyond a reasonable doubt, and overcome the explanations that the defendant will offer.