If Sejuani really needs to have a stacking passive, change the stacks required to 6 stacks, but have Sejuani add 2 stacks per hit; this creates a greater incentive to hit things herself, while still letting allies get in on the stun stacking action.
I'd like to see her fitting in any comp. So she won't be with "Gnar, [Insert any mid mage's], ADC, [Insert any range support] and be like "well guess I have to stack it on my own." So maybe some changes so Sejuani / Melee stack it easily but ranged stacked for half/third of it.
I'd personally like to see the ally interaction completely removed and make Sejuani able to stack her E up with all of her abilities. 1 stack on R impact (for whole AOE), 1 stack for R explosion and 1 stack for Q. She gets back some of her lost teamfight threat by having stronger options for AOE stacking and loses the dumb ally interaction which only serves to detract from her kit.
I do agree that if the ally interaction has to stay something like what you suggest would certainly be better than leaving it in the state it's in now. The absolute bare minimum change IMO would be to force Sej to be the one to initiate the stacking.
Or make her Ult apply a stack to all enemies hit
