Arelith

While manacled, I attempted to cast a number of hostile spells to break free, but I was still afflicted by 100% spell failure even though I was applying the still metamagic to all my spells

I was under the impression that this was the entire point of still spell, this exact situation and armor.

Please make it so casters with still spell can cast while manacled

I think it is auto still spell that unlocks that feature. More and more spell failure reduction with more auto feats. You can test on the PGCC.