Onan - interestingly enough, an examiner is either PIC or not, but the decision is retrospective! On a successful flight test, the candidate logs P1. Unsuccessful he logs dual instruction. I'm only a humble destructor, and don't know how the examiner would log it.
For all formal training the instructor is PIC, even if the student is qualified to fly that type in the prevailing conditions. Otherwise the instructor wouldn't be able to log hours!
Notam - I am often the PIC of a PA28 in the right seat. I can reach the fuel selector without too much trouble, although I prefer not to so I brief students to do it very early in the course, reminding them in the pre take-off brief of the actions required if I ask them to shut down the engine in the event of an EFATO. Given an EFATO on a trial lesson I would probably leave fuel on until we were on the ground, use the mixture to prevent engine restart and limit fire. Not ideal, but safest I can get.