If you could go back in time and speculate on what this would look like had you met him shortly after they opened up their marriage ( and without illness) how would things look today? It might be impossible to remove the illness because it plays such a large role in this but give it a shot.
Who or how .....was the date time decided?
Her shift in attitude towards him maybe out of self preservation ....oh fuck I could actually lose my nurse/piss boy if I keep this up ...I better be nicer to him this is getting serious. Which could explain the daulity of her behavior in a group setting verses the colder persona when along with you.