a/ Almost never, Engineers tow the Jets as required.
b/ Driver in the Tug ( obviously !! ) and an approved person in the cockpit on the brakes if required in an emergency to stop the Jet.
Sunfish:----you must be joking surely? Of course they can do Engine runs!!
Engine runs with pax or crew on board require the CN to sit in a control seat.( not to run the Op but to look after any EVAC situation as it occurs )
If the ginger beers need to do an Engine run ( and they do often ) with no crew or pax on board then they obviously don't need a crew member present as they are big boys ( or girls ) and can do it on their own quite well
