Peace can also mean lasting ceasefire with no deaths for 6 months.

Doesn't have to involve the exact borders of Crimea, can include other territories, but at least 80% of the population and territory of Crimea.

Doesn't have to be a state as long as it's not under direct control of either Ukraine or Russia.

Can allow for a limited amount of troops for either side, for the purpose of inspection or passage, especially if the base of Sevastopol remains leased to Russia; but only if kept in check by a much larger multinational peacekeeper force.