I'm concerned that if I cut A and B from this tree that the small branch between them would die. I'm considering developing that small branch as the new apex.
What is your opinion? I believe there is an issue with sap not continuing to flow to a small branch when a large branch near it is cut? Is this true?
The answer to your question is, "not necessarily". If you removed branches A and B, the branch remaining (D in this photo) would not necessarily die. You'd want to leave stubs that are about twice as long as the width of the branches to help ensure the new leader reroutes any water lines over the next few months, and carve them away over the winter.
Now...do you want to take a longer view of this project? Real opinion? The area you circled as "C" is your tree. Everything above it is sacrifice branch. Develop the upright green arrow as the new leader (allow 2 buds to grow, 1 gets to run free and be a sacrifice branch, one needs to be restrained to keep short internodes). Do the same with te branch marked with the left-pointing green arrow as the first branch.
With your proposed leader, you'd be lucky to achieve the first image in 8 years; but likely even more abrupt transitions from one trunk section to the next. It will be tall, awkward, and stiff, with no low branches.
With my proposed plan, you'll likely achieve the second image in 5-6 years, and have a more compact structure to refine over the the next 3-4 years.
Ultimately, it's your tree, but at the moment, you have options. I think the second tree has much more potential.