You did understand that the music has a specific programme, right?
The only way to lengthen the 3rd movement would be to lengthen each of the initial sections. I had considered this at one point, but feel that it meets the requirements of the programme at this length in this form. Altering the form by adding what you call a "b section" would undo the form and alter the programmatic intent (by the way, adding a section would make it an "f section", seeing as it already has a, b, c, d, and e sections!
).
The next movement (the 2nd adagio) will ALSO be relatively short.
The final movement, will be longer.
I also needed to look at the over-all length of the work as a consideration.
Thank-you for taking time to listen to my work.