I suspect you are not actually considering multiple modifiers. Multiple "more" or "less" modifiers are always multiplicative with each other.
A modifer which says "x% more [stat] per Y" is not multiple modifiers, it is one "more" modifier, with a value that varies based on Y.
The new Immortal Call has exactly two modifiers to Physical Damage Taken. One of them has a static value (for a given level of IC) and the other has a value which is modified per endurance charge consumed. Regardless of the number of charges consumed, there are still exactly two "less" modifiers to physical damage taken. These two modifiers are multiplicative with each other, because they are "less" modifiers.
The second one using "per" to detemine a variable value for the modifier, does not make it into more than one modifier.
EDIT: corrected my description of the modifiers on IC to "less" damage taken rather than "more". That'd be quite a nerf :P