I have checked all the things you mention except 1/4″ tubes inside 1″. What is 1/4″tubes inside 1″ and where is that? Thanks.
I have done a test with a cigarette butt.
When I flush, the water goes into the bowl, the cigarette butt will only swirl on top of the water. When the water level become low then the cigarette butt will enter the passageway but it comes back.