My 'Napoleon's Infantry Handbook' deals with various issues relating to firing. A couple of comments you might find useful having read the thread:
Firstly, the forming of two ranks in 1813 does appear to be a one off 'ruse' rather than a tactical change. The French were quite conservative really, and were still fighting as Frederick the Great had taught them. I have seen a similar thing in Spain when a regiment wanted to appear bigger than was the case and so formed into two ranks.
Bardin describes a platoon (peleton) as having a minimum of 12 files. This is very small - I've not seen anything which says you should form two ranks if you fall below certain losses. Do not forget the platoons are equalised every day so loses would be evenly spread across a battalion (which is why you should not call them companies - a platoon can have men from different companies in it).
Bardin says the British were better at firing than the French because they practiced musketry more. He believed the French had the best musket, but they did not teach soldiers how to aim.