Yeltsin getting shot on the tank isn't impossible but seems unlikely to me – the man was a secular saint in 1991, after all.
@Callan and I have discussed the far more likely possibility of Gorbachev getting shot by a trigger-happy guard during his little Black Sea holiday.
With regards to a civil war, well, you might have folks like Albert Makashov (that is, literal neo-Stalinists) taking up arms to preserve the Union as it is and restore it to the way it once was, but I doubt many people are actually going to do that! Certainly the Soviet breakup will be a lot messier in Russia than historically, and the political wounds of whatever form of civil war results will be long, long lasting, but I don't think said civil war will be particularly conventional – not a lot of pitched battles, more urban militias and guerrilla warfare, and, honestly, a bit lower-key than you might think. I really doubt you have any nukes fired, honestly, unless, again, a Makashov gets his hands on one.