It is interesting that you bring this up I was just explaining this problem.... I lived in South Dakota and during a particularly cold spell (it did not get above minus 20 degrees F., for a week) Most of the homes had the vent pipes freeze over, which causes the sinks and toilets to drain with much difficulty. The problem is caused by a lack of airflow behind the exiting water through the waste lines. You can reduce the effect by using a toilet on the lower levels, of a home, as the airflow needed will be drawn from the sinks in the upper levels. Additional problems with that approach is that you will notice a "sewer gas" smell coming from the upper bathroom sinks, since the water in the traps, of those sinks, has been reduced based on the suction caused from the bathrooms below.
Yes if the vent pipe is 4" and vertical by code and there is nothing connected between your toilet tee and where you would connect your new toilet tee. If not worried about codes then yes you can connect to a 3" vent line if no other vent lines are connected down stream. If you have a vent pipe that is less then 3" and or you have other vents connected between your old toilet tee and your new toilet tee then NO you can not connect a new tee for your toilet.
Yes as ALL fixtures need to have the vent placed above the FLOOD LEVEL RIM
No
Who needs a toilet
Within 24 "
Yes.
partially clogged drain or vent, improperly installed vent or drain
Within 24 "
Any vent is usually the size of the drain or waste pipe it is connected to. 3 or 4 inch on a toilet.
Should be individually vented
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.
Yes. You need to open the plumb, & you have an air vent there.