Doctor Who show runner Steven Moffat has confirmed where in River Song’s timeline the 2015 Xmas special is placed - directly after she’s watched the Eleventh Doctor say goodbye to the Ponds in The Angels Take Manhattan. I’ve updated my great big River Song timeline graphic to include this new information. Mrs Song certainly does get around!
Oh my gosh you guys I just realized that when Melody asked who River is, the Doctor says "Spoilers" because he's remembering all those other times he met River, but that's the first time she meets him, so all those times she says "spoilers" she's quoting him, and all those times he says "spoilers" he's quoting her!!!! <-- Mind. Blown.