i also think at least in the beginning she wasn't seen as the major contributor that she has become. She always talks about how in the beginning she was just happy to be there and perhaps somewhat exaggeratedly says that because she was just a "chick singer" and not a real musician she was in somewhat of a "lesser" position. So I can see this being a conscious or unconscious factor for the first album and even for Rumours because wasn't OMH a bigger hit than Rhiannon charting-wise? I think Christine was obviously the senior songwriter from FM and Lindsey being the songwriter/arranger/producer on BN material he had more clout in the beginning than Stevie.
And in those days she deferred a lot to his decisions on which of her songs to include. Then they just got into this 3 song thing and it became her mantra......kinda like the "we break for 3 years" thing in recent years. After Rumours and Tusk I agree she may have been holding onto certain songs for solo work, though she's always claimed that she shows them everything and they can use whichever they want.