Hi,
I'm a new player, and I'm not very familiar with the game's earlier stages of evolution.
I am trying to build a melee 2-H build, since many guides are obselete ( gate of fates is totally different than many youtube guides show etc. ) I was trying to do some theory crafting myself, and currently I'm a bit in a bump since I'm not sure about mechanics.

My main gap is on understanding damge increase, I noticed it's inconsistent with additive / multiplicative bonuses:
I've noticed that sometimes damage bonuses / penalties are additive and sometimes are multiplicative but I can't figure how and when exactly:

let's say I have the following bonuses from passive nodes and items:
1. Physical damage: +15%, +15% + 10%
2. Rend damage: +15%, +15%
3. Material damage: +8%, +9%, +12%, +11%
4. Fire dmage: +15%, +10%
5. Melee damage: +5%, +10%, +10%
6. Two handed damage: +8%, +8%, +5%
7. Attack damage: +10%, +10%, +5%
8. Critial damage: +7%, +8%, +30%, +5%, +10%, +15%, -50%
9. Damage: +10%, +15%, +15%
10. 50% of physical damage is converted to rend damage.

and let's assume that all of my skills are tagged as 'attack'.
and let's assume that my weapon looks like this:

100 physical damage
50 rend damage
50 fire damage

I assumed that for each category the damage increase is additive, so lets say we can sum the bonuses above.
Now should I assume that each of the above damage type increases will add up ( additively ) to correspond with the associated base damage?

for example:
fire damage gets + 25%, physical gets +40% and rend gets +30%. Now I also have for two handed dmage bonus of 21%, does it stack up
1. additvely for each dmage type? ( 46% fire, 61% physical, 51% rend )
2. or a total of 21% damage boost on top of that? ( 1.21 * ( fire+physical+rend ) )

Same question can be asked for other damage increase modifiers ( like material damage, total damage and so on.. ).

The reason I'm not sure is because I tried to remove and add some passive points and I mostly saw it's additive for most nodes, but for example, I tried to use the 'Retaliator' bonus node from the 'Praetorian' tree, some bonus damage increase based on resistance and -50% critical damage.
And what I noticed is that when I take it, I drop from 270% critical damage to 135% critical damage ( when I expected it to be 220% ), so as I said for me I'm still not sure when it's going to be additive and when it's going to be multiplied, and it's getting complicated when so many modifiers are involve ( I listed just 10 above, sometimes there are more ).

If anyone from the developers or if anyone has some solid knowledge on how it works, I'd appiciate the help.
Actually if someone can help me understand what will be my expected weapon damage with the base damage and the modifiers I gave as an example this would help the best.