Simone - the viaduct is, in fact, open to any kind of traffic, but by common consent no-one in the village uses a car there except in dire emergency - pedestrians rule; only the local farmer who has land and animals on both sides of the Dochart uses a vehicle, and he may well own the bridge; he certainly owns a lot of the old railway line which crosses it.
Francis - CruachanPS, Oban, Stalker - in that order - would be possible in a day - morning at the PS, lunch and afternoon in Oban, and Cas.Stalker in the evening (it's an evening location anyway). Tight, but possible.
Kilchurn is not for the some day, I think. There is quite some walking to get good camera positions - you would need an hour for each camera position, plus you might have to wait for good light.