I will have to answer that all later when have more time, but first off you can't speed things up by splitting the bank using the same chargers.
If you have four batts and two 40ampers, you can put both on the four or one on each pair and it will take the same time because your charging rate is the same in each case 40 on two is the same percentage of AH as 80 is on four.
the fastest absorption to follow the75 when it drops to 13.6 would be with a 75a charger that would start at the 14.6v and taper its amps down that curve the same as the 75 would have if only it had stayed at 14.6.
Anything less than 75 will take longer.
You can get a faster charge from 50-70 by using higher amps than 75. Just toss in another charger like a 40 and do it at 115 instead of 75. You still need to do the 70 on up while amps taper but you are starting tapering from 115 which just changed to 40 if that was the second charger's amps, so you can add some more chargers--the only limit is how much generator you have to run that many chargers at once.
I'll get back to your scenario later.
