Zinc oxide reduction in ethanol
As for your first question, I have personally had better luck only aging 24-48 hours (actually unsealed, just covered with Kimwipe to prevent contamination, but allowing for some of the solution to evaporate).  We're getting good $\ce{ZnO}$ XRD peaks on this process, and are currently developing an SOP/Optimization for $\ce{ZnO}$ by sol-gel technique. I'll let you know when we publish.

When you say you're getting $\ce{ZnO}$ and Zinc peaks, do you mean you're seeing XRD peaks matching Zinc as crystalline metal?  To your question precisely: "What possible reactions would produce $\ce{Zn}$ from $\ce{ZnO}$, and how likely are they?" Remember that your sol-gel is not necessarily $\ce{ZnO}$ yet.  The reaction that produces $\ce{ZnO}$ begins with the dissociation of $\ce{Zn}^{2+}$ and $\ce2{CH_3COO}^{-}$ ions in ethanol, followed by a hydrolysis. As to how you're getting the extra electron to precipitate metal, I don't know. What we have seen has been the precipitation of $\ce{Zn(CH_3COO)_2}$ after about 96-120 hours after originally mixing.  (We're pretty sure it's acetate because it smells like vinegar when we wash it out of the flasks). There is a very good article going over the chemistry and technique of $\ce{ZnO}$ sol-gel: article