Wow...you really wrote a lot here......well, the thing is I'm in a war with the mamlucks, the supply limit sucks and they have cav double the infantry.......even thou I almost have triple of the forces I should easily win BUT....everytime we battle I lost not less than 7-9K inf....since you say at tech 13 the cannons are almost useless I'll leave them sieging, and now I'm thinking, the infantry put in the second row do no damage, right? but can they actually (during the battle) come to the first row if a unit gets completly destroyed? And if not, or this doesn't really happen often why should I put the inf behind?
To answer you question i'll divide it.
Artillery start being effective at 13 specifically. They are still under powered because of the HUGE upgrade at 16. Depending on who I am playing I will have art in my comp at 13. If its the Ottomans... always. if it was an OPM in a hard start... well i might not be able to afford them. If I can I do. Maybe not every stack but i do.
As to inf from the back row going to the front. It doesn't need the annihilation of the unit (stack wipes are normally moral base not actual casualties). If the unit routes (the individual inf unit), the unit behind will step forward in its slot. Units in the back row do 50% damage. a unit in the back row is generally better then not engaged at all (in reserves sitting taking moral damage) or just not there, as they will do damage.
What nation are you playing? Ottomans? This might help answer some specific questions to your issue. also, game date, ideas, their current ideas etc. Screen shots help.
Also Mamluks like Calvary. Their ideas support it with a 10% Cav combat ability at game start. Them being Sunni (more horses in army before inssufuciant support) and quite wealthy at start allows them to field a hard hitting army in the early game. Like as Ottos, I normally try and hit at tech 6 to their 5 (maybe after patch it will be 7 vs 6 due to the no reclaiming cores and having to spend admin and delaying -20 core cost) on the first war for the Lavant, which swings huge in my favor. This normally guts them hard enough and by the second war my army is that much bigger that I can roll them over (also I try and ignore Damascus in favor of Karak). Also having multiple units near each other going down the Lavant. I find this can work vs ottos late game also as it is not very wide and the forts are not in mountains (except Karak) and you can re-enforce heavily.
Lastly another big thing is going to be depleted troops. If they are not at full strength they will take a pounding. Swap some of the info from back line support units to keep the line full (I just swap inf units around as my stacks are almost always similar like 10/4/5 or 10/4/10 for capitals with 10/4/0 support units. I do this for almost all nations. I like fielding whole "units" , but yeah some start prohibit this as mentioned above. Or shift consolidate to have more full powered units.
.... wow wall of text.