Not really Iran will crumble under pressure if anything it will be war to eliminate them. Saudi has many allies in the region to take care of matters. Iran will have to fire back at Saudi arabia thats when you will see neighbours invading Iran from all corners. Saudi is holy land and war on the holy land is war on all Muslims. This will not go well for Iran. Turkey and Pakistan will have to by default intervene on behalf of Saudi Arabia. The holy land belongs to all muslims and the protection of it is for all and required by all muslims
'Saudi' is not holy land, 'Hijaz' is. The traditional guardian is the Sharif of Mecca, currently King Abdullah of Jordan. The family of Saud are considerd to be usurpers.
Nobody is 'compelled' to come to the aid of the Saudi regime, muslim or otherwise. Standard realpolitik applies. A war on the Saudis will never be considered a war on Islam.