Russia soldiers without insignia have invaded and fought in Ukraine before under the direction of the Kremlin. These were the infamous “little green men” who occupied Crimea. Russia originally denied their involvement, only acknowledging them after Crimea had been fully taken. There’s every reason to think that Russian backed separatists in the Donbass region are likely getting the same sort of help.

Ukraine is losing population, yes. The problem is that, as you can see from your graph, it has been doing so across every government. The post-Euromaiden protests government is far from special in this regard. In fact, the only difference it has from previous governments is that it had a slice of its land cut off by Russia, so in that sense it “lost” extra population.