PRAISE THE SUN!
Okay, got that out of my system. Part of the problem here is that we really don't know what Zunism was. Paradox made them pagans to add another challenging start, and to to add another pagan Asian religion aside from Tengri. However, CK2 Zunists run on rule of cool as well as the grossly incandescent power of the giver of justice. It was most likely a combination of aspects of a Hephthalite god, Ahura Mazda and the Hindu sun deity Surya, probably leaning more towards Surya than the other two. It was less of a religion and more of a cult built on top of several pre-existing traditions in the region at the time.
However, getting this cult to survive is possible. I'd suggest having the Arabs win the 674 siege of Constantinople, or at least beat the Byzantines so badly that they empire gives up Anatolia alongside the rest of its Asian territory. Either way is going to get rid of the biggest opponent of Arab expansion in the Mediterranean and open up the riches of the black sea, which could be more enticing than a mountainous desert much further away. Alternatively you could crush the Berber Revolt before it starts, which could keep the Umayyads both focused on the west and in power. A third possibility is a far more destructive Riddah War that shatters the Caliphate in the early period. The Muslims are still powerful, but now they'd be fighting each other as well as the infidels.
As for 'reforming' the faith? I think that your best bet is actually to have more Buddhists in the region- perhaps they are driven to Zunbil after one of the Buddhist kingdoms to their west falls. Buddhism, as we've seen IOTL, is pretty good at incorporating the gods and beliefs of the native cultures into Buddhist teaching. And if Zun is another name for a Hindu god then it's even easier. This is probably the only way a Zunist 'reformation' could actually happen. Otherwise it's probably too entwined with the Zunbil dynasty to stick around after they leave.
