[Christ]

#1 and #2 are never accurate at the same time, hence the caveat, #3.

Depending the situation, it may be more efficient to use the highest gear and load the engine as much as possible, whole another situation might warrant leaving the car in a lower gear, allowing the engine to rev to it's peak bsfc area (usually where max tq occurs, also) before shifting.

Specifically, if you need to accelerate quickly, shifting at peak torque, same with uphill.
However, it's generally considered more efficient to ignore the bsfc curve and keep the engine loaded at let rpm, as fuel consumption is based on time(rpm) and power. The idea is to use the least power you can, at full throttle, so that LOD is as close to 100% as possible all the time. Technically, the closer you are to 100%LOD, the more efficient you're being with the fuel being burned.