Who is to say that Mexican energy can't trade, without hinderance, to Ukraine directly? It's possible. And maybe probable if American LNG funneled through Pemex's subsidiaries to Ukraine is completely legal.
It may be too late for the U.S. administration to salvage Crimea in the short term. But crafting an energy strategy that bolsters future geopolitical security for Europe and Ukraine would exploit the homemade crevasse in Putin's armor.