No.
I would not expect a valve cover leak to cause a car to stall. A valve cover leak can fill the spark plug area with oil which can cause rough running.
YES, That would be called a vacuum leak.
Yes, that is one of many possibilities.
You may have a leak of coolant. Take your car to a shop and tell them you are smelling coolant and may have a leak.
Probably a coolant leak.
The coolant leak is pretty cut and dry - you have a leak somewhere. The lukewarm heat (assuming you're referring to the car's heater) could be for a number of reasons. Insufficient coolant could be one of those. Your coolant could be burnt out. It could be a problem with one of your heater hoses. You could have a worn out heater core.
There are many things that would cause a 1999 Chevy Lumina to stall. The car could be too low on fuel, have a leak in the fuel line, or an old fuel pump or filter.
could be steam from a leak caused by the accident in the coolant system or smoke from a transmission cooler leak they are both up in the front of the car.
no, but having a vacuum leak at the power booster will. IF so, you will have to replace the booster.
Yes but it would have to be pretty large and lean codes would precede it illuminating CEL/MIL
A bad freeze plug will leak coolant and cause the engine to overheat.