You seem to want or need to have a person single-handedly responsible for the shift from heavier-than-air flight to lighter-than-air flight.
I'd like to suggest an alternative approach which may give you the result you require.
Back in the 1970s there was a some fairly serious talk in some circles about a possible resurgence in the popularity of blimps and dirigibles for long-distance travel. The reason? The Energy Crisis which began in October of 1973.
Currently we are undergoing a climate crisis based off the use of fossil fuels, and oil-based energy continues to become more expensive as we continue to burn through irreplaceable reserves.
Additionally, consider that while airborne transportation is very popular with over 100,000 flights worldwide per day, it seems to be increasingly difficult to turn a consistent profit from year to year in that industry. Wikipedia lists 83 airlines, since 1979, which have gone out of business, re-organized as another business, or have merged with a larger airline.
You'd have to work out the timeline, but I would suggest a future where it is simply too expensive for most commercial endeavors to employ heavier-than-air flight in their daily operations. One man has seen the writing on the wall and has positioned his considerable wealth into the re-development of dirigible and blimp technology. During the crash of oil reserves his gamble pays off and he's now the leader of some dirigible monopoly.
Dirigibles would now occupy the place of the ocean-going liners of the late 19th and early 20th century as the primary mode of international transport. A 1930s dirigible crossing of the Atlantic took from 3 to 4.5 days, depending on weather and routing and such; I would suggest that with modern technology and knowledge of atmospherics one could probably guarantee a crossing in either direction in 2-3 days.
An envelope calculation figures that it would take about 1.16x10^12 cubic feet of helium to carry the same amount of cargo as a supermax vessel, so you may want to consider a similar, parallel, ocean-going sail industry! It currently takes a cargo vessel 9 or 10 days to cross the Atlantic -- sometimes as long as 20 days, depending on the actual destination. My own limited experience with ocean-going sailing makes me believe that a modern cargo vessel with the best sail technology would take a good month to cross the Atlantic. So: maybe the majority of cargo is moved around via ocean-going sailing vessels, but you'd still have some class of super-dirigible for carrying cargo more quickly.
Helium without an oil-based economy is an interesting problem. We don't have a lot of helium in the atmosphere because it's so light; it floats to the top of the atmosphere and is blown off by the solar wind. 308
My understanding is that we get most of our helium from the oil industry. So if you want your dirigibles to be elevated by helium, you're going to have to find a source.
I would suggest a world which uses fusion energy. It's a source of energy which would be only economically feasible for city-scale power production, but which produces helium as a "waste" product. Now you have a cheap source of helium. But, fusion power would be too expensive, not to mention too bulky, to power even a very large cargo vessel, much less an aircraft.
Now, while hydrogen has proven historically to be a bad choice as the lifting gas for dirigibles, I would suggest that is only true using 1930s technology. Using modern technology and materials the use of hydrogen as a lifting gas would arguably be much safer, with the added benefit that you can use it as a fuel for propelling the dirigible. So, if it's a story you're writing, or an RPG milieu, you may want to use hydrogen as the lifting gas to introduce some kind of balance or limiting feature in your world, especially if you really don't want to use fusion technology.
And the use of dirigibles would not preclude the use of fossil fuels in other areas. The military, of course, would continue to use fossil fuels in nearly all their aircraft. Private and government aircraft would probably use jets powered by liquefied hydrogen.
Those are my thoughts on this interesting question. Hope that helps. 435