To revert to foxhotís original question, Iíve been considering a whole new option just recently: Kirk went back to Ceti Alpha V. My scenario is that "Space Seed" happens fairly early on in the 5-year mission (late July or August 2266 depending on how I finalise things, stardates 3141 to 3143) and then the Enterprise makes a return visit, towards the end of the original mission (maybe March or April, 2269, before TAS, when Chekovís still definitely aboard, stardate around 5550). This allows Khan to remember Chekov without him having to hide somewhere on the ship for the first year of the show. It means that the "fifteen years" that both Khan and Kirk say has elapsed since their last meeting takes us to the mid-2280s, rather than the early 2280s. (Iím thinking of placing Star Trek II in April 2285, to match up with nuTrek putting Kirkís birth in 2233.04, and also to fit with the death of Joran Belar on stardate 8615.2, 86 years before "Equilibrium". The movies III-V would follow Star Trek II between April and August 2285.)
My guess is that the Ceti eels seem to be a desert creature (they live in sand), and they wouldnít really come into contact with the settlers until the planet started turning into a desert, after the Ceti Alpha VI explosion, Iíd put that six months after the second visit, September or October 2269, approximately stardate 6050. Iíd kind of assumed that none of the children born on Ceti Alpha V survived, and only the youngest of the frozen supermen made it. Theyíre all blond and pale-skinned because the alkaline chemicals in the Ceti sand have bleached the colour out of their hair and skin. (Well, at least it completely and totally avoids the whole "people with yellow hair and pale skins are somehow superior" nonsense.)
So my "out-of-nowhere" suggestion is that Marla McGivers dies about a year after the explosion of Ceti Alpha VI, September or October 2270, stardate around 6950. Khan is eaten up with hate because Kirk did return, once, then never again. The dialogue doesnít really support this theory, but (at least as far as Iím concerned) it doesnít absolutely rule it out, either.
Of course, if Kirk goes back MUCH later, say 2275, then itíd be 2290 when Star Trek II and III happen, making sense of Admiral Morrowís comment that the Enterprise is 20 years old. Unfortunately, that doesnít leave time for Captain Suluís thrilling mission to survey gaseous anomalies before Star Trek VI. I still havenít thought of an answer to that one.
Last, Iíd just like to say Iíve read and enjoyed Greg Coxís books about Khan. So Foxhot, if you want to stick close to stuff thatís been in the official books and the established continuity, Iíd follow what he says, and ignore my idle speculations. Cannon? Wasnít he that umÖ burly detective?