[Question] Delevel Skill

How this work???
I am not asking how to do, this I now.
Exemple:

Can I pick up

Light Mastery 90
Sub-skill's light lv 90

after that delevel the mastery to 0 and have the sub-skill light 90? how works?

I am not too sure what you mean.
I believe you are asking if you can delevel the mastery past a skills level.
The answer is no, you must first delevel the skill before you can delevel the mastery level.

I will test to make sure my theory is correct, but I believe everything must be deleved in order to bring down a mastery's level.

Yeah you're right, you have to delvl the skill before the mastery.