I suppose in theory, if you somehow got hands on a fused version of the eyes like Sasuke has, it would degrade to a rinnegan only. Just a theory though, could be very well it just blinds the whole thing regardless, or simply doesn't work at all.
What comes to Danzo's eye closing, personally I suspect it may have been simply a result of excessive gene manipulation, a timer of sorts if you will, so he could keep track of his get outta death free jutsu. If my recollection that Madara's eye turned out the same as Itachi's after the transcription seal version, a hazy whited out one, is actually correct then I daresay that's the result of a legitimate Izanagi/Izanami in any case, while Danzo's was an unique case.
Lastly, since I know there are those who switch the whole eye between rinnegan and sharingan states at will, I'd say if you use either jutsu while its in sharingan state then its bye bye to both eyes, can't swap it to rinnegan anymore again ever without getting a whole new one.