Ishar also has former dealings with Odium, and isn't exactly an example of upright moral character
Ishar is completly crazy,
Even with Dalinar the Stormfather acts like an odd mash up between force and man.
The Stormfather has had multiple bondsmiths and mentioned that there were some men he genuinly enjoyed, he is more than aware of the cost of immortality.
That feels forced.
As the bond with Dalinar progresses the Stormfather becomes more contemplative.
Additionally the speaking pattern doesn't match Ishar at all. Where is the bragging, the egotism, the complete and utter belief in his own infalibility?
Honor forces the Stormfather to give the visions
Lift can enter the visions but cannot start or end them
Odium cannot start or end the visions, and is you know, a shard.
The Nohadon vision that wasn't even a vision, and at most you could call it a dream based around it.
It was Nale who we saw the Oathpact from. And even with his Blade Ishar was unsure if Kalak had died, so why is he suddenly better at feeling the Oathpact without his blade?
The Stormfather has absorbed Tanavast's cognitive shadow so he definatly has access to the Oathpact.
Ishar is crazy, the vast majority of the time he is dead set on becoming Adonalsium, not making new Heralds. None of the other Heralds experience such a dramatic change. In fact Kalak seems better in RoW than he was in the prologue.
The Stormfather has stated multiple times that the Oathpact was shattered, so why waste time fixing what is irrepairibly broken.
I see no reason Ishar would be the only Herald to change in behavior.