If you are coherent enough to tell us about all this I would say that there was little to no damage. It depends upon how long the brain went without oxygen. The rule of thumb is this. Every minute that goes by you are without oxygen the is a 10% loss of life. Brain damage starts to happen at between 3-4 minutes.
Possibly. Not necessarily In general, it depends upon the duration of the oxygen deprivation, but that is subject to caveats. In the case of an opiate overdose, the body is essentially unconscious and oxygen demands of the extremities are diminished.
Sometimes an MRI/MRA will show the electronic signature of an area of dead tissue,