How do different sources of minus enemy defense interact?

Hi ♥
I'm using the act1 cold merc for uber support on my warlock (Mist,

Kira's Guardian, Duress). Her level 32

Inner Sight (-1815 defense) along with level 39

Death Mark (-1380 defense) and level 21

Sigil Lethargy (-53% defense) means my

Bind Demon has no issue hitting any of them.

Uber Baal should be reduced to 0 defense from

Inner Sight and

Death Mark alone but i'm wondering if anyone knows specifically how these all work together?

Does -% apply before flat reduction, or after? I assume it's after, but if it's before it means next respec i'm definitely going with 1 pt into

Death Mark and finally ditching goatmen all together. Or potentially switching to act 2 might merc with Pride (but he's quite hard to keep alive) since i won't need

Inner Sight

How does -% target defense from weapons fit into all of this? Does

Sigil Lethargy stack with it or is it more like how

Critical Strike / deadly strike work?

Thanks^^