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Both dishwasher and toilet are sharing the same outlet pipe/drain it is possible when disahwasher is in use it is blocking of the drain somehow so the toilet water has nowhere to go - you need to have a look at the outlet pipe of the toilet and dishwasher the problem lies in a conflict there.
the plumbing drain vent is not installed or poorly installed.
Same as a regular toilet.
It is called a back outlet toilet floor mounted
30 inches.
If your plumbing system is properly installed there should not be any problem. If it isn't installed properly, you would already have water from sinks and bathtubs backing up out of the downstairs toilet.
Every toilet should be wet vented with the sink in the bathroom.
Medical supply places should have them.
Homes are residential and take less of a pounding compared to the constant use of commercial toilets in office buildings. Commercial toilets are built way stronger and usually have to be sized according to handicap rules in the city where they are installed. They also take a 1" water supply compared to a 1/2" supply for a house toilet.
yes.
Toilet parts
No, toilet seats are not hard to install. There are two bolts that go through the rear of the opening on the toilet and when these bolts are secured, the seat is installed.