One thing is for sure... you have coolant in your oil.
It depends on the engine, but some intake gaskets can also seal coolant passages. The accelerator issues sure sound like a blown intake gasket. A quick way to find out is to run the car and spray a little carb cleaner around the intake manifold and see if the RPM climbs. If it does, the intake gasket is bad.
I'm surprised your mechanic just do some quick tests to verify.