I don't have my camera handy or I'd try a few things and see if I could replicate your issue. Try playing with the speed dial on the back, and make sure it is mating securely to the body. What are the chances the new battery isn't good?
One thing I do know - sometimes my flash won't fire (on the speed grip), and/or will fire on advance, and the shutter will act weird - but only with one back. (The back also tends to open randomly (the catch is bad), so it's now a parts-back).
It's possible the back can cause issues on an otherwise functioning body.
I forget how to dry-fire, but I think the mirror lock might be involved.
Make sure the multiple exposure lever is not set for multiple, though I don't know if that would cause the problem in the finder.
Also, check and make sure your lens isn't set for the "timed" exposure, which I believe is pulling or sliding a pin on the barrel near the front of the lens. Again, not sure if this would cause your issue.