I think we will just have to agree to disagree about this

I think more or less everything that might have happened did actually happen. To my mind, the idea that Dean was boxed in by Jan's condition is a revisionist myth.
True, Dean recorded SFARD while waiting to see if Jan would recover and then caught a lot of flak about it. After that, though, he led several projects, any one of which could (with a bit of luck) have resulted in a top twenty hit and a new career.
He was present at some of the Smile sessions and, whether or not he actually participated, he was close enough to have recorded and released a version of Vegetables (with Brian Wilson on piano) before the Beach Boys did.
A few years later he started the Legendary Masked Surfers (which also included Brian Wilson, Bruce Johnston and Terry Melcher) with the idea of having some fun and seeing if there was any potential for a bigger long-term project.
LMS released two singles and then (deciding that the potential was indeed there) morphed into California with the intention of becoming some kind of post-surf supergroup for the seventies.
These projects show Dean involved on his own with A-List Californian musicians and immersed in aspects of what would become the seventies west coast sound. These could have resulted in hits and a new musical career. They didn't- but that is not because Dean didn't try or was somehow constrained by Jan's condition.
California, in particular, started out as a serious enterprise although, as far as I know, nothing came out of it and it eventually morphed again in California Music which released a single or two (and had a CD compilation released on a small label a few years ago). If they had seen it through and produced an album that shone they we might all think of Bruce, Terry and Dean somewhat differently now.
By the way, does anyone know if the original participants in California actually got anything down on tape? Is there any unreleased material that is more or less finished, or was California just a daydream? (I will make this a new topic to avoid confusion!)
Whether it was a daydream or not, the record clearly shows that Dean was an active participant in a rapidly changing and evolving music scene *before* he decided / agreed to continue working with Jan. I am not sure how this would have been different if Jan had been dead or permanently retired.
Cheers
Owen