My interpretation of the book is that the Defense Professor looks just like Quirrell. If this is the case, then maybe it takes more and more out of him to maintain the illusion that he is someone else. Or maybe he actually inhabits the body Quirrell, and Quirrell is slowly fighting back.
Then again, I still have a hard time reading the DP as actually being Voldemort, so take my instincts with a grain of salt.
I think you are erring when you assume that these are Voldemort’s plans. They might be, but I don’t think they have to be. The story seems to have deviated quite far from the original story.
In fact, my reading is that Quirrell may actually be some good guy, destroying our expectations from the story. I mean, has his turban even been mentioned?