The Donbas secessionists want to leave because the Maidan coup started pushing the country towards Ukrainian ethnonationalism, and eastern Ukraine is very Russian. Russia did back them, but Putin did not fabricate a popular opposition to the Maidan government.
Denazification is a pretty clear objective when the entity that you are seeking to denazify has battalions that are openly and explicitly Nazi as part of their doctrine. Purge those units, prosecute former members, ban Nazi hate crimes (like greasing bullets with pig fat) and ban Nazi symbols like Swastikas and Wolfsangles. It’s really not difficult, the US military nearly passes that bar and even the German military does (though it has closeted Nazi cliques).