Lets say your right and that he would still by their constitution, be considered their president. the president does not have the power via their constitution to invite foreign powers military into their boarders, so he is guilty of treason. Also he has basically stepped down from his position in favor of being a member of an illegitimate government set up by a foreign power. So of his own volition he is no longer a member of their formal government, regardless of weather the parliament can remove him without the constitutional court.
Second he is guilty of mass murder... this action by its self makes him an illegitimate leader by Democratic and Liberal standards.
Third, russia has annexed land from GEORGIA... or did you miss that? its a de facto annexation, so its not official... but it is there. Then there is russia's actions in regards to the Ukraine... sending additional troops, flying helicopter patrols over unapproved( Via the lease agreement they have) land, and by sending unmarked fully armed and trained russian troops onto a foreign national and sovereign soil in an attempt to set up a puppet government because they didn't like how the parliament had removed their former puppet from power, pending a constitutional court decision(This whole PENDING part is the constitutional question).
As for the constitutional power that the parliament has in Ukraine, it is highly debated weather the parliament can temporarily remove someone who is charged with these crimes from power until a constitutional court decision. its about a 5050 split in the countries rather short constitutional history.