There are definitely peak-null-peak-null positions as you go from floor to ceiling. So would that suggest a mode between ceiling and floor?
That plus the fact that this pattern is the same throughout the room....
Yes, 2nd Height Mode. It is however close to others.
One trap overhead will do little but do include an airgap to help.
The corner traps would benefit from being in the floor or ceiling tri corner.
But Floor to Ceiling is my minimum recommendation.
If you could position your woofers in that height null, the mode will be less strongly driven and the differences will be way less. Consider the speaker upside down or even sideways, whatever it takes.
Good catch and onward. DD