There is more than one way to plumb this but you basically need to run a vent up from the vanity drain into the attic and tie it into the vent going through the roof.
You need to have a licensed plumber install that water heater and you need to get a permit and have it inspected, if you don't
and it causes a fire your home owners insurance wont cover it.
It has nothing to do with " THE CAR RUNNING INTO IT" The water is elevated to keep the pilot light from igniting any gas that is spilled
on the garage floor.
Dig up that clean out by the front patio and see what you got. depending how deep it goes you could tie right into the drop. I hope you have 4" pipe going to the city sewer because when your done i'm guessing you'll have more than two toilets on your property.
I wouldn't buy that for a second. you need to have something done to correct that immediately, before it's to late. I don't care who say's that's acceptable.
If it doesn't flood now that you've had the drain jetted, don't loose to much sleep. most times jetting solve's the problem.
if the problem keeps coming back then yes the drain is sagging but it's not an emergency and it's something you can plan for.
When he sent a camera down the drain did he...
The fittings you are talking about are called " PRO-PRESS " their quicker to install than sweated joints and their great for repairing copper pipes when you can't get all of the water out of the line. but their expensive. Pro's use them all the time.