It wont hurt anything to leave it open even with the furniture over it, but if you want, you can take a towel and plug up the airflow from the vent. This will only make the other vents in the house blow a little more air than normal. just take the vent cover and put it back into place over the towel.
See: How Safe are Vent Less Heaters
There should not be a vent penetration that close to a valley.
It has a vent, but it is not for over filling.
As close as possible is best, but there just has to be a vent so that the air that is in the line has somewhere to go so that the liquid isn't trying to compress it. The closer the vent is to the toilet, the better it will flush.
Usually at the top or close to the top. It is linked to the vent (above the vent) where magma erupts.
Yes
They get food
I would say ok on the supply but not hte return.
no it has a vent if it is coming out the vent it is over full.
WHICH VENT If you're talking plumbing roof vents or soffit or ridge attic air vents; absolutely not!! I can't thing of a single vent that would be opened and closed at will.
call 911
drivers door has a switch for each window