Katy, why do you not want to have the trees. I think they look nice, it's part of the motif. Sky is about all you can fill it in with but you have to be mindful of the shading. I have made minor adjustments in my time but in this case I would leave it as is. Of course you can finish this by November. A couple hours each day will do it. Just start in one place and continue on. I agree with Nancye that "jumping" all over the pattern causes problems.
I admire all stitchers that can think about Christmas before Spring has even started. There is no way that I could do anything as ambitious as this.