Your examples don't consider the fact that Mexico 68 was in altitude, and NEVER some athlete born and living at sea level (and training in altitude for some short period only) can have the same difference of a Kenyan in the performance comparing the two situations.
Athletes born, living and normally training at sea level go to high altitude in order to improve their ability FOR COMPETING AT SEA LEVEL, but their adaptation in altitude, looking at the best possible performance, also if can improve with prolonged stays in altitude, NEVER can allow them to reach the same level of performances of a Kenyan, who at sea level can run at the same speed.
Repeated periods of training in altitude produce a big improvement in the level of adaptation.
I can give you here a clear example, regarding Salvatore Antibo.
He started to train in altitude in Sestriere, in 1988. The last workout before returning sea level was always the same : 4 times 2000m with 4 min recovery, plus 1 time 1000m at his max speed.
In 1988, before moving to Seoul Olympics, he ran at the average of 5'45" the 2000m, and in 2'37" the final km, and the Italian Coaches were surprised (myself included), because the idea we had about the difference between times at sea level and at 2050m of altitude was, at that level, something about 10" - 12" per km, so that training seemed amazing.
Antibo went in Seoul, and won silver with the new National Record of 27'23".
In 1992, Antibo was no longer at the same level, for some problem he developed after an accident in 1990 in October, after his best season (when really he had the possibility to run under 27').
However, before Barcelona, the average of his 4 x 2000m was 5'28", and his last km 2'31".
Between 5'45" and 5'28" there is a difference of 7"5 per km, that in 10000m means 1'25". But it's clear that Antibo NEVER could have had the same advantage he had in 1988, because 27'23" - 1'25" means 25'58". So, the explanation was a great improvement in ADAPTATION to altitude, with less influence to the result at sea level.
Before Mexico, nobody of the best athletes had real adaptation at the unusual situation, and all the best European, American and from Oceania, could run only at a reduced percentage of their real value.
The bigger case was Ron Clarke, who was clearly the best at sea level, and almost died in 10000m. And I think that Jim Ryun (in my opinion superior to Keino in 1500m) suffered particularly the altitude, not only under physiological point of view, but also under mental point of view, since his interpretation of the race was something very wrong, leaving many athletes between him and Keino, with a pace slower than his final time.
About Ben Jipcho, it's not possible to compare his result in Mexico 68 with the result in Christchurch 74. At first, there are almost 6 years of difference, and in 6 years Jipcho improved a lot every where : he never was able to run under 4' the mile at the time of Mexico (in 1969 he ran 4:00.4), and was clearly there only for pacing Keino.
About some doping at that period, for Kenyan athletes, I'm really very much doubtful. There was no organization in Kenya in 1968, not only as Federation, but also for common citizens : a big lack of medicines for curing the diseases and the sicknesses of the period, a high level of young mortality, problems to find enough food..... Really, I can't see a picture of athletes thinking of, and using, doping, in a period when the main problem was to survive (like happened in Ethiopia some year later).