Also a civilian who has struggled with understanding how this stuff worked. Changes since the end of the war, variations for Guards, Rifles etc. units just make it more complex. So with a grain of salt, my understanding for the Second World War as follows:
- Acting Rank had the pay and allowances of the rank
- Brevet Rank did not have the pay and allowances of the rank (I've never encountered a real example of a Brevet rank
- Appointment to an acting rank could be temporary (either by planning, by unsuitability of the soldier, by request of the soldier) or could be made permanent through confirmation in the higher rank. I've heard 3 months as the period in which you had to do the job before being confirmed.
- L/Sgt was also used by Rifle regiments, with half the detachments in the A/Tk and Mortar platoons being commanded by a L/Sgt (the other half by a Sgt)
Bear in mind that in the Second World War, rank structure within a unit was fixed (ie. there were a static number of Cpls allowed), so you only got promoted to fill a gap in a higher rank. In a rifle platoon, a man promoted to Cpl was promoted in order to lead a section, not simply to reflect his experience, etc. While I am sure it is not entirely correct, my broad understanding of the current CF is that ranks like Cpl or M/Cpl are as reflective of time in as much as responsibility, though I admit to ignorance of how the intersection of seniority and responsibility is resolved.