Well lets get this first post here started right!
Perhaps the current on Reverse Mountain … reverses occasionally We've only 'seen' it at one point in the day, right? Seems to be there's the possibility of that, a la: low/high tide.
I apologize because if what I'm saying has been covered, but I would like to try and work this out as well and straighten myself out as well if I am wrong. I can't for the life of me find a picture of any of the 'maps' that have been shown of the grand line near the time the Straw Hats entered.
Garp also travelled to East Blue when he picked up Koby and Helmeppo, and during that episode I recall the announcements saying that they travelled Reverse Mountain on their way back. There are a couple of problems with trying to figure this out, though, in that we don't know whether he was at the Marine HQ beforehand, nor do we have any frame of reference for time since they just 'immediately' arrived back at HQ.
The better example, as stated earlier, is Mihawk following Don Krieg back to Barartie. If I recall right, the Red Line crosses both poles of the One Piece world and divides it into a 'western' and 'eastern' hemisphere. So both parties would have had to either traverse Reverse Mountain, or the equivalent at the other side of the Grand Line. And since Don Krieg and Co. turned back, Mihawk probably wouldn't go all the way around the world to follow him, right?
In the end, all that I am left with is my original hypothesis, in that the current changes at times and travel is allowed to the Blue's from the Grand Line at Reverse Mountain as well.
Wow that got wordy, sorry for anyone up this late at night trying to read all that, I've never posted here and I guess this really just struck a creative nerve for me.
EDIT: I think I got a little mixed up on which 'part' of the Grand Line the story is currently in. I'm recalling now that East Blue should be the sea to the north of the Grand Line they are in now, right? While typing that for some reason I was thinking they had the Red Line dividing where they are and East Blue, and not a Calm Belt. Either way, I suppose the hypothesis can still stand.