Drudge is highlighting there was tension between DOJ and FBI agent. I expect DOJ made a deal with Mills to which FBI is not a party. If Comey resigns, it will be to support the integrity of the investigation. It would be like Watergate's Saturday Night Massacre.
The most obvious reason for the presser is probably the correct one - which would be that Comey just wants to quell rumors and speculation by stating that the investigation is ongoing and they aren’t going to be rushed into completing it for political reasons.
They haven’t interviewed Shrillary yet, supposedly, so how can there be any decisions about referring charges to the DOJ yet?
Comey probably won’t say anything specific about the investigation, just that it is ongoing. We’re going to hear a lot of, “I can’t speak to specifics since this is an ongoing investigation.”
“There was Cowboy Neal at the wheel of the bus to Never Never Land..”