had such a small window to launch
He had even smaller window to launch, because lots of shell crates was unloaded to ground near artillery to make Gaza out of Donetsk (for "final solution of Donbass problem"). In a couple of days it would be hundreds of thousands of dead civilians, in addition to 14000+ that was killed in previous 8 years.
And do not tell about "territorial integrity" for ukraine, read UN Resolution № 2625 (XXV) dated 24 oct 1970. The principle of territorial integrity is considered there in relation to states that "respect in their actions the principle of equal rights and self-determination of peoples <...> and, as a consequence, have governments that represent, without distinction of race, creed or color, all the people living in the territory". Consequently, it can be concluded that the principle of territorial integrity is inapplicable to states that do not represent all peoples living on their territory, fail to ensure equal rights of the peoples living in it, and do not allow free self-determination of such peoples. Moreover, this declaration, while directly prohibiting actions leading to the dissolution of states, does not prohibit the recognition of new states that have unilaterally seceded from other states.
After a Maidan coup, nazi from the west that comes in the power definitely not represented people of Crimea, Donbass and Novorossia, and state terror that followed attempts of self-determination (terror was successful in Odessa and Kharkov but failed in Donbass) clearly showed that russian majority in that regions was oppressed by new government.