Here’s a summary of intl law related to Russia’s involvement with Ukraine. in 2008 the Intl Criminal Court (ICJ) decided that Kosovo’s declaration of independence was legal, though it didn’t rule on the actual act of secession or the recognition by other countries of Kosovo. It implied that separatists are not obliged to respect the territorial integrity of the state, only other states are thusly obliged under intl law. The declaration is legal, even when a Security Council Resolution on interim administration and settlement process is in effect. The signatories of Kosovo’s declaration didn’t act as the same Kosovar representatives subject to the SC resolution, but did act in an interstitial moment between subjectivity to it and realisation of statehood. This precedent dissuades Ukraine from taking Russia to the ICJ over Crimea.
Ukraine has taken Russia to the ICJ under the Intl Conv. for the Suppression of Financing Terrorism and Convention on the Elimination of Racial Discrimination (CERD), for Donbass and Crimea respectively. Ukraine claims the rebels’ “indiscriminate shelling” is terrorism, as is MH17. They claim Russia is pursuing a policy of “erasure” of Ukrainian and Tatar culture in Crimea, limiting media and language education, banning the Mejlis and failing to investigate missing activist cases. The Georgia-Russia ICJ case on CERD is relevant to this. Georgia charged Russia with ethnic cleansing but the ICJ threw that case out out as no attempt to negotiate or consult the CERD committee had been made prior to applying to the court. Ukraine hasn’t appealed to the CERD committee either.
Ukraine’s case appears based more on hysterical innuendo than fact. For Crimea, Russia has the Dec. 2016 UN High Commissioner for Human Rights report, CERD committee review, and ECHR coverage on its side, none of which raise issues covered by CERD. In the terrorism case, it has OSCE reports. No state or intl organization has referred to rebel acts as terrorism, even MH17. OSCE reports show more victims of Ukraine’s indiscriminate shelling than of the rebels’. The Mejlis has explicitly threatened and acted to cause humanitarian catastrophe in Crimea, eg. by participating in bombing the superconductor towers serving it.
There are a lot of other technical details to the case, but in neither the Donbass nor Crimea has Ukraine the evidence and confidence to raise the issue of violation of territorial integrity or right to self determination before the ICJ. To its demerit, Ukraine insists that a BBC report showing tanks on the site of a rebel shelling not be officially included in the case as it exposes their deceptive tactics before the court. They also bizarrely claim that the rebel republics are not signatories of the Minsk II Accords, and that Ukraine never agreed to amnesty, which is I think article 5 of those accords. I wouldn’t bet money on Ukraine here.