Why is the DOJ focused upon these minor issues compared to Comey’s certification of the FISA warrants under penalty of perjury?
Because we strain at gnats and swallow camels. FISA warrants are beyond the average person's pay grade, but they can readily understand the threat meant by 8647.
Maybe the same reason they got Capone on tax charges
This might be a place holder event and DOJ is letting Comey know it...
Perhaps this will provide support for assertion of a "Continuing Criminal Enterprise" that extends the statue and allows re-examination of this the perjury associated with the FISAs declarations, that, and a more favorable jurisdiction in which to file a criminal case.