Dont get me wrong, I can only speak for myself, but - Ive had symptoms of sinusitis when doing the exercises (as well as when being exposed to the harmful displays) and also eventual tightness and cracking in my jaw (very loud one). I got checked for sinusitis and also for jaw problems, sinusitis is negative and Ill get more examinations for jaw. However, since the only significant find is the heterophoria, and since its a very rare and completely under-described phenomenon, I believe it might be that feeling of full sinuses is simply when the muscles (which are in the sinus area) get overworked, and the jaw muscles might tighten due to an attempt of the whole area to keep tight those eye muscles, that due to the heterophoria, cannot keep tight (transferred muscle tension). All these things are well known in other parts of the body with muscle problems (broken toe can transfer to muscle pain in the pelvis, as whole system is out of balance and balances it out with previously unused muscles, or unnatural ones for the task).
And since many of you already have a find in the eye-muscle area, Id blame that and incompetent specialists that are not so far able to help you completely.
I progress now at a steady pace and I really notice week to week slow but noticeable differences in tolerance. Its like Im training to run a marathon and can run a bit longer every time. However Im not sure if ill be able to run with no end (work on a large problematic display for many hours). I will see.