No way they have everyone working on online. Everyone work on specific things, what they are good at. Not everybody generally working. Not all of them are either authorized or set on the online work.
Exactly. Two days ago they made the statement that "their entire team is working on it" -- referring to GTAO.
But the majority of the issues they are dealing with right now are server related, not technical. The two week wait for the online, coupled with the astronomical sales numbers served as a lethal duo for their servers as they were quickly overwhelmed by the hundreds of thousands, possibly millions of people who were trying to simultaneously connect. Server related issues of this kind are much easier to resolve than say technical issues with the game itself.
They have been looking for and purchasing hundreds of more servers like crazy since experiencing the October 1st servers crisis. And I imagine that since the greatest issues at the moment are server related, other members from different departments inside the dev crew were requested to help out with this endeavor as well. But this doesn't mean that EVERY single person in their division is fixated on the online component.
It was confirmed by Lesie Benzies that GTA IV had nearly 300 people working on it full time, and somewhere between 600 - 700 people working on it all together either full time or part time from their Edinburgh office , Leeds, San Diego, NY, etc., so I'd imagine GTA V has even more people working on it considering how big a project it is. I highly doubt that literally all 300 - 400 of them are working solely on GTAO right now as we speak.