Can someone please help me understand why I'm not being permitted to equip this other spear in Weapon Set II? All attribute requirements are met, there are no skill conflicts, and my slot I spear is not shared.

Are you sure attribute requirements are met after weapon swap as well?

In patch 0.4.0 at the start of the league, many Augments (socketables) were changed to be "Limited to: 1". Whether as a bug or by design, this restriction applies across weapon sets. Mouse over each of your Augments to see if you have a duplicate Augment that is Limited to: 1 shared across your weapon sets.

Yeah man I'm way above the attribute requirements, and I'm currently level 86.