In another update on the WWE World Heavyweight Title situation at WrestleMania 31, it's now being reported that Brock Lesnar knew when he signed his new deal with WWE that he would not leave the event as Champion and was most likely informed about what exactly was going to happen with Seth Rollins cashing in his MITB contract. Lesnar, Paul Heyman and Roman Reigns spent a lot of time working together in the days before the show so those three at least knew what was going to happen.
When exactly Seth Rollins was informed of the plan is still uncertain. There were initial reports that stated that Rollins didn't even know about it until the show was well underway, but that may or may not have been the case.
As for the reaction backstage, while there aren't any reports about people being upset that Rollins left as the Champion, there are people who feel bad for Roman Reigns and are being defensive of him. Reigns is said to be well-liked and fairly respected backstage and doesn't have very many problems with people. Some people backstage blame the vocal portion of the audience for seemingly killing Roman's push, while other people backstage aren't necessarily mad at the fans but still feel bad for Reigns over the whole situation.