Your scenario - blame West Germany for a terrorist act to spark war with NATO, then invade the Middle East for oil - reminds me a lot of Red Storm Rising. Not that I don´t think it´s an interesting scenario to investigate from a post-Cold-War point of view, though.
How about the casus belli is faked "West German terrorist attacks" against East Germany? Say, the "West Germans" try to assassinate an East German government that shows itself too resistant to the idea of more openness and closer relations to the West? That, and they are found "supporting violent counterrevolutionary elements" in several Eastern Bloc countries.
Then, while the crisis slowly escalates because West Germany denies everything and fake "West Germans" continue to act against the Soviet Union, some kind of coup like the 1991 one removes Gorbachev from power because he is "too weak to resolve this crisis" (because the crisis is being caused by the coup plotters).
How does that sound to you?
Yeah, since I've read that book several times I can't deny being influenced by it.
Hmm. I like the idea of the causus belli being NATO support for 'violent counterrevolutionary elements' which are killing off NSWP leaders; it really brings the NSWP nations on board with the war and culls off some of the democratization movements therin at the same time. Nice.
I still think Gorbachev needs to die, not just be removed. In OTL he had enough influence to stop the coup; if he's dead no one can argue that he has any influence left. Plus if he dies in the midst of a bunch of terrorist attacks on NSWP member states... "the counterrevolutionaries could be anywhere! See? They have murdered our beloved Comrade Gorbachev! To arms, workers and peasants!"