Like I said, if it's before water7 even started then Oda was probably referring to Usopp who left while they were in water7.
Also, I'm sure we'll get some crazy explaination about how the Going Merry got there in one of the next two chapters, for now we can only speculate. I think they've actually been on Enis Lobby for a while, there's no way for us to tell day and night because Enis Lobby is always sunny (unless my memory is completely wrong and I'm pulling that outta my ass). There's no way to assemble that amount of battleships in less than a day.