There's an old joke;
"Doc, it hurts whenever I do this."
Doctor says, "Don't do this. That'll be fifty dollars."
I don't clearly understand the palate/sinus phenomenon that you are describing, but if you can sing nicely in tune while training yourself to consciously 'keep it open', then continue to train that way, and everything will be fine.
Singing does NOT come from your diaphragm. Your diaphragm regulates your breath. Singing uses your larynx, epiglottis sphincter and intrinsic and extrinsic laryngeal muscle and cartilage. Do more research on the larynx and you will find your answer.
I am a vocal specialist with a BA and Masters in this field and have been teaching for 8 years as well as ...