Or a hydrogen atom, which electronic transition would result in the emission of a photon with the highest energy?

This electronic transition would result in the emission of a photon with the highest energy: 4p – 2s This can be the same with the emission of 4f to 2s which would emit energy in the visible region. The energy in the visible region would emit more energy than in the infrared region which makes this emission to have the highest energy.

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2