*nods* which is why I kind-asked if TWF/other MAPs factored into "voluntary penalties".
I'm reasonably certain the TWF penalties are not what the designer had in mind for "voluntary penalties". I think this refers to the penalties in feats like Power Attack, Deadly Aim, or Combat Expertise, where the description explicitly says "You can choose to take a penalty ...".
The wording is very similar to Blade Tutor's Spirit, so it would work in a similar fashion. Power Attack, Combat Expertise, etc. Not Two Weapon Fighting. Combination Feats rely on having/landing multiple attacks, so lowering penalties on later iteratives/flurries makes sense.
Almost thread necromancy, dear Yuri. Depending on mods.
Use your [Hex Spell] Vulnerability on them, and they they are now vulnerable to Hexes more than once... including [Hex Spell] Hex Vulnerability itself. The only thing you lose out on is CL to overcome SR and Saves, neither of which matters for a beneficial Hex since your ally will simply forgo their saves.
Meaning, so long as you keep hitting them with [Hex Spell] Hex Vulnerability and never let it drop, you're good to go for the whole dang day of infinite beneficial hexes.
If it ever DOES drop, just be sure to keep Hex Vulnerability prepared as a 1st Level Spell to get the gravy train rolling again. Downside of course is the low Duration of 1 rd/level, meaning it requires rather frequent refreshing.