You have asked some very good questions that show a solid understanding of the physiological aspects of endurance. I hope this information will be of use and interest.
1) FT vs ST muscle fiber composition. It is much easier and with better results to turn a FT dominate athlete into a good endurance athlete than it is to turn a ST dominate person into a speed and power type. I have personally done this with nationally competitive athletes. They might not have had the genetic gifts to become world class but they were able to rise from rather mediocre results to national championships. Heikki Rusko did a longitudinal study of young Finnish Cross Country skiers who were engaged in high level training. Over the study duration for about 10 years he saw a significant increase in ST percentage. As far as I know this is the only longitudinal study like this on humans but many animal studies have been done with the same results.
2) Here at Uphill Athlete we have coached hundreds of athletes with relatively poorly developed aerobic systems. Universally we see improvements in aerobic capacity as evidenced but increased pace at AeT. We have also had thousands of people apply our training methodology either by reading our books or buying one of our training plans. Every week we get several emails from this group telling us of their individual improvement in AeT pace. It is very common to see these increases be in the 20-30% range after 6 months to a year.
3) Improvements in AeT pace in the submax effort levels used in long to ultra long duration events we cater to leads to a performance improvement. While a person with a very high maxVO2 will have greater capacity for hard work by virtue of having a an higher anaerobic threshold the performance benefit of this will be much less in these ultra long duration events than in, say a Cross Country ski race of 50km or less or 10km running race or VK where absolute max power will play a much bigger role in performance. In these ultra events other factors than maxVO2 weigh more heavily on performance
In the end, for each individual, maxVO2 is irrelevant. It is what it is and there is very little you can do to change it. So focus your efforts on what you can change and most cases change by double digit improvements. As a reasonably fit adult you will never see double digit improvements in maxVO2 until you get old and then it will be double digit decreases!
You might want to read this: https://uphillathlete.com/max-vo2-myth/
I hope this helps.