Looking forward for more input on this topic.
Just some information concerning your last paragraph:
There are basically 4 different ways to "stun" an enemy. - Fear: enemies cower on the floor, for example Boromirs Horn
- Terror: enemies run away cowering, for example the Screech of the Nazgul
- Paralyze: enemies are stunned, for example Soul Freeze
- Slow: the movement speed is set to 0, for example Lurtz Cripple Strike
The normal Fear Resistance (Spellbook Horn, Faramir's leadership) offers resistance against Fear and Terror, but both can be gained independently in theory. Paralysations can't be countered at all and Slows can be only be countered by their anti category.
Every modifier has a type STUN, DEBUFF, BUFF, LEVEL , STRUCTURE , WEAPON, SPELL, FORMATION , LEADERSHIP. These are used for different types of modifiers, but have no direct effect as far as I know. Also every aura can list an Anticategory for the modifier it offers. If the Anticategory is active the modifier won't work. Exactly this system is used for Freezing Rain.
The problem is that the Anticategory only works for Auras (standard leaderships), not for modifiers in weapons ore active abilities. Sadly these are the ways 99% of all Stuns and Slows with Modifiers work. I guess it would theoretically be possible to rework all Slows such that they work with Eggs which are spawned and have an Aura (similar to the River of Mirkwood if I remember it correctly), but this would have to be Areas of Stuns/Slows. Then Lurtz Cripple Shot might target nearby enemy heroes, too if they are too close.
Therefore, immunity to stuns in general is not possible, which makes your concept obsolete. Applying only the resistance against Fear and Terror in the way you proposed is probably also either impossible or hardly possible, because the immunity has to be applied delayed.