One way to check the volume issue (non-math) is to push / rock the buggy while you slowly apply the brakes. Then take note of the pedal travel needed to just barely apply or drag the brakes. If it used up most of the pedal stroke, you have a volume issue. If you still have a lot of pedal movement left, you have some other type of issue. This can also help you to determine whether you have brake line flex or not.
The brake lines will not swell when just a little bit of force is applied at the pedal. However, when you try and stop and put a lot of force on the pedal, this is when they will swell. Remember that the brake lines are seeing 200 psi and up (in a hard stop). If your brake lines are moving around, then they are swelling. A hydraulic line will become more rigid when it is pressurized, but it shouldn’t move or flop around. Also, if I remember right, the piston size on my Hammer Head are 1/2" and they control both of the front calipers fine.
Since you have all the time in the world and the money as well
, your best solution may be to run a long piece of steel tubing back to a tee and then use short piece of flexible line to each wheel. That is if the above test shows that you have enough pedal movement.