Mon Mothma is easy to explain.
The one that is/would be hard to explain is Biggs.
If that is Biggs then we do actually have a continuity problem on our hands. When Luke runs into him at Tosche Station he states his intentions of jumping ship and trying to join the Rebellion. He doesn’t supposedly join the Rebellion until sometime after that before Luke makes it to the Hidden Base.
Now, some will say that it’s a deleted scene and therefore doesn’t count. HOWEVER, it’s been officially stated that the novelizations of the films are canon and that scene is included in the novelization of Star Wars.
Theories?
Well, in the old EU, Biggs was lying to Luke and had already joined the Rebellion months earlier. He essentially described to Luke what he had already done, but pretended that it was something he was planning to do so that he could both “recruit” Luke without potentially compromising himself or the Rebels to the Empire.
It’s possible that this idea will be re-canonized (unless of course the novelization blatantly contradicts this, which I wouldn’t know as I haven’t read it.)
EDIT: Are the novelizations really part of the Disney canon? There’s a fair amount of PT contradictions in those books, especially in ROTJ.