I don't think the bartender dude would have gone along with Joe if he knew that they weren't going to bury the bodies, so maybe Joe's going to the cemetary to get bartender dude to participate, remember his surprise when he finds out their not going to bury the bodies....
I always thought the cemetery was chosen because it was a good distance away from the town. Joe wants to have everyone, particularly Ramon, his brothers and most of his men, a good distance away from the town so he can poke around the Rojos to find out where the gold was being kept. Seemed to be one of his main motivations in setting up that event, besides restarting conflict between the two families after their dinner. The town is referred at one point as a cemetery. So I agree there's the contrast of the cemetery for the living and dead. To have the two factions go against each other in the cemetery underscores this in the film. I would agree that the cemetery is probably close to the river massacre site. Seems like the river site, cemetery and safe house were a ways from the town. True, the bartender, Silvanito, helps out thinking he and Joe were going to bury the bodies.