I didn't read all of your posts, but I had this issues in my G6. Sunroof was leaking awful, turned out the hoses that are connected to the drains came unhooked.
I was blowing them out just as you mentioned you did. That's apparently wrong and you are supposed to only use the special tool they have to clear obstructions to prevent the hoses from coming unhooked.

Sorry if that's already been mentioned.