Why would anyone do that? Steffen was a GOP operative that they wanted to get. I don't see any possible advantage to "frame" O'Doherty, though I would not be surprised if he claimed that.
The use of the first mail drop shows an intent to hide his identity. The use of the regular email was most likely laziness or possibly because he forgot the password to the maildrop or, more stupidly, used the FR password for the mail drop and wanted to delete it.
I believe that O'Doherty could be covering for an even bigger operative ... his new job might just be a reward for keeping his mouth shut.