I really despise the “R2 downloading the map from the Death Star” explanation. My headcanon is that Luke intended for Lor San Tekka to bring Rey into the fold at some point like Obi-Wan did with Luke and send her to Leia with the map. R2 was programmed to wake up when Rey registered on whatever sensors he’s got going on while he’s in standby mode, hence why he only comes to life when Rey arrives on D’Qar. Lor San Tekka entrusts the map to Poe at the beginning of the film because he sees that the shit is about to hit the fan, and either he wasn’t aware Rey’s presence being necessary for R2 to work or he intended to reach out to her after the fact and send her to the Resistance if Kylo hadn’t killed him. It isn’t airtight, but it makes some amount of sense for Tekka to ditch the original plan to keep the map out of enemy hands.