What is the clear criterion for Ta'wil (allegorical interpretation), and is it authentically reported that any of the scholars excommunicated the Ash'aris for their saying "istawla" (He dominated/seized) instead of "istawa" (He rose over/ascended)?

There is no difference between excuse by ignorance and excuse by interpretation (ta'wil); in fact, the one who interprets is more deserving of excuse. However, this does not prevent punishing him or fighting him to دفع ضرره (repel his harm). Not all interpretations are permissible; interpretation concerning the essentials of religion is clear disbelief. The acceptable interpretation is that which does not nullify the religion, conforms to language and science, and its proponent intends the truth. The hadith about the splitting of the Ummah indicates that most sects are not disbelievers, and the one who is outside the fold of the faith is the hypocritical disbeliever. If scholars ruled the people of innovation (Ahl al-Bid'ah) to be disbelievers, they meant disbelief that does not expel one from the faith, or they intended to warn against the saying. Scholars differentiate between ruling on the type and on the individual when declaring people of innovation to be disbelievers; they rule the belief to be disbelief, but they do not apply it to a specific person until the conditions are met. The Ash'aris are not outside of Islam; they are among those excused by interpretation. Whoever denies what the Messenger of Allah, peace and blessings be upon him, brought, or part of it, without interpretation, is a disbeliever. The people of innovation are of different categories: some are disbelievers, some are misguided, sinful innovators, and some are less than that, like the Ash'aris.