The tiny speed envelope at max altitude -is not peculiar to the U2-
Check with any guys who drive commercial jet stuff.
the window can get pretty tight.
(I know this is not a model thing -just commenting )
as far as fretting over the airfoil - -just make it as equal as possible from wing panel to wing panel.
On this type planform (aircraft as viewed from above)--the real evil doer will be the DIFFERENCE in airspeed between wing tips.
One of my first experiences in RC modeling -35 years ago - was a model laid out in a very similar setup -- close coupled - small stab.
It was extremely easy to yaw the model at low / landing speeds -OR in a slight crosswind - trying to visually level the model-- get a difference in tip speeds .
the result was always a drop of the slower panel- and correction was always impossible .
The lateral area on the U2 makes yaw control touchy -- we do this on some models to make them good in wild aerobatics - but in this setup - I would sure make sure I had the landings and takeoffs well up to speed so that the plane is ON the ground anytime speed is below positive control on all surfaces

great looking project.