They already did… https://scontent-iad3-1.cdninstagram.com/t51.2885-15/e35/12747577_1664945460421434_1798709718_n.jpg
The helmet in RO even looks as unpolished as in ANH next to ESB.
Well, if that is from RO, they really screwed up. In both Rebels and ANH, Vader’s shoulder armor is covered by the robe he wears. It appears under the shoulder armor in ROTS, TESB, and ROTJ, but if you are presenting this as immediately prior to ANH, it has to be under the shoulder armor or it is a mistake.
As for Mon Mothma, they don’t really need to explain anything. She appears to be on the 4th moon of Yavin in RO and she was not there by the time the Millenium Falcon landed. There is quite a bit of time in between that includes the Emperor disolving the Senate and the destruction of Alderaan, not to mention the success of the the plot to steal the Death Star plans. We don’t need to be handed every piece of information on a platter in order for the story to make sense. Her reason for leaving the rebel base is irrelevant. We know she did because she is not there later. Likely she returned to Coruscant to protest disolving the Senate or she was just there to set the mission in motion. She likely couldn’t just hide out at the rebel base and needed to get back to work. And it is obvious from the larger Alliance in ROTJ that someone was working on gaining new allies and it makes sense that it would be her. All this talk of her being in RO being a continuity error is nonsense. If you want real continuity errors, the OT has them in abundance but most of us just ignore them (or make fan edits that fix them).