Have not read the whole thread, however:
The more i read the rules, the more I'm convinced that staking effect is one of the trickiest part.
things as to hit and to wound cannot exceed +1 or -1, but you can stack 2 sources of +1 to hit, to overcome a malus.
(source: page 4, bottom left: "some dice rolls, such as hit and wound rolls, will specify that the roll cannot be modified by more than +1 or -1. When this is the case, add up all the modifiers that apply, and if the total is more than +1 or -1, treat it as being either +1 or -1 as appropriate."
so, if your stegadon chief is charging something that gives a -1 to hit, makes sense to use a Their Finest Hour (heroic action) combined with All out attack (Command Ability).
by this reasoning, the same can be said for bonuses to saves.
A save roll cannot be modified by more than +1, but can be modified by more than -1.
If i stack bonuses to have a +2 to saves (Their finest hour + All out defence), i will only have a max +1
However, rend modifies my save roll.
If i have a +1 save, a rend -3 will impose a -2 to my save.
but if i have a +2 to save, a rend -3 will impose just a -1
Stacking different sources of the same bonus, even to things that cannot go over +1 / -1, is going to be crucial.