It was the opposition usurping power using assault rifles at Maidan, Feb 2014, that was illegal
Arguably, but they were responding to a democratically elected leader deliberately contradicting the platform on which he'd been elected against the express will of the people. Either way, that's a purely internal Ukrainian matter, not something for Russia to get itself involved with.
not Odessa, Kharkov, Lugansk and Donetsk regions declaring independence after those events.
Not 'after those events', but rather 'after occupation by a hostile foreign military force'. Apart from Russia, is there anyone that accepts that the 'referenda' leading to those declarations was free and fair? The UN certainly doesn't - see
here.
who had refused to recognise Donbass as two autonomous states within Ukraine as the Minsk agreement stipulated they should
That would be the Minsk agreement (actually Minsk II, given the complete failure of the first) that said control of the borders reverted to the Ukrainian authority, and which Russia continued to send personnel over coordinating with their sponsored stooges in the contested regions? That Minsk agreeement? The one that included a ceasefire that the 'rebel' forces in Donbass and Luhansk never complied with, that Minsk agreement? That Minks agreement which, again, is a PURELY INTERNAL UKRAINIAN MATTER.
What terrorists and what smokescreen excuse?
See above about a freely elected president (unlike the first time he 'won' when the result was overturned because of widespread corruption and voter intimidation) who turns his back on the will of the public to favour his (presumably) personal ties with Russia, then the continued identification by groups such as the European Parliament, the US security services and the British intelligence apparatus of Russian state sponsorship of terrorist activities, including arming and maintaining militias in Ukraine and the continued illegal occupation of the Crimean peninsula.
O.