The engine management 'sees' the leak as open throttle and richens the mixture. If you had to go through annual emissions testing you would have more then likely failed. You probably blow the O2 sensors clean often enough
to keep them from sooting up and malfunctioning, they would have tripped the check engine light (called a 'MIL' these days - Malfunction Indicator Lamp.)
The EGR valve introduces inert gas into the combustion process allowing timing to be run more advanced without worry of detonation. It also helps reduce emissions as such. Normally EGR valves are only opened in third and fourth gear at cruise. Yes, opening it at idle will kill the engine, so if it were stuck open the engine would stall at idle. Ask someone with a 4.3 Blazer, it's the most common bad EGR valve, these days...
Disconnecting it would be poor form, besides, if it were that easy don't you think the factory would have left it off? "Gee, let's put this valve on here and steal 65 horsepower..."
Beav