Is there a mod that changes how SP and HP reduction passives work?

It's exactly as it sounds. So SP and HP cost reduction passives are applied AFTER you select a skill, meaning you need to have the base cost of the skill.

Example: Allure of Wisdom reduces SP cost for magical attacks by 75%, lets say a magic skill costs 100 SP and that gets reduced to 25 SP, HOWEVER, you need that 100 SP to even use the skill, it doesn't make sense in the slightest.