Your instincts are correct, yes you can own all of the indulgences, but their effects are not independent. If you can “keep up to 5%” and “keep up to 20%”, 20% is as much as you can keep.
tbh the stacking rule was a necessary addition for other purchases to work how I wanted, and the cathedral purchases have been a little confusing ever since. Saying “up to X%” was my best way of simply saying “you get this much and no more, regardless of which other indulgences you have.”

































