A recent article on the "Fact Trek" blog tackled the question of why the character of Marla McGivers didn't return in Star Trek II. It took on the rumor that supposedly the reason was because Madlyn Rhue was already well-advanced with MS and thus, after featuring the character in early drafts, the decision was made to eliminate her.
The blog notes that this story doesn't hold up, as there is no citation for it other than the constant "telephone game" of someone repeating a rumor. It notes that Rhue, while she had been diagnosed with MS as far back as 1977 kept her condition hidden until 1987 and until it became more serious she was still actively working in TV even at the time of WOK (she even appeared on "Fantasy Island" just after WOK came out). While she was moving more slowly she was still getting by at that point so her illness wasn't the reason for her not appearing.
After showing all the excerpts from early script drafts that featured the character of McGivers (misspelled "McGiver") a new interview with Nicholas Meyer confirmed that it was his decision to make Khan driven by revenge for his wife being dead. The blog also notes that at this stage they still hadn't bothered to check on Ricardon Montalban's availability so that indicates that Rhue (who apparently never once gave an interview about Trek) was likely never approached.
The biggest gem from this feature though is the inclusion of an audio excerpt from the WOK workprint that shows that McGivers was referenced by name in the Ceti Alpha V scene by Khan but the line didn't survive to the final cut and only this audio exists. They should have left it in.
Wow! This is totally awesome. Learn something new everyday.
To be honest I don't mind this bit of dialogue being cut out. Khan does mention the death of his wife, Star Trek fans know who he's speaking of. The general audience doesn't really need to know her name or backstory.