anyone over the age of 50 will have many many things disallowed due to pre-existing conditions. When I fell in a castle ruin and broke my elbow and required surgery, I had a lot of hassles about whether a pre-existing condition was involved -- I am thinking, 'yeah right, I got on a plane 5 weeks ago and flew to Russia with an elbow in 6 pieces and then waited till I got to the south of France to treat it.' What I was told is that some falls are caused by pre-existing conditions and thus would be excluded. Because I got the policy immediately when the first outlay for the trip was made, it was irrelevant.
I do not think you can use the 60 day trip covered by Medigap policies and then add a 30 day policy for the last 90 days of a longer trip. The policy wording basically indicates that for coverage to work, the entire trip has to be covered. Since the profit model for all insurance companies is 'deny coverage whenever possible' you don't want to make it easy for them to exclude uyou. I asked that question when buying insurance since our medigap covers 60 days and was told that the coverage would not work unless it covered the whole trip.
We now have an annual Allianz policy that covers 90 days as well as a medical evacuation policy. It is all very expensive at our age and we hope not to have to use them. The single trip coverage I had 6 years ago from Allianz worked very well so I hope their annual policy does as well.