Lactose will absolutely raise your FG, for exactly the reason you suggest. Why don't you run the numbers through beer calculus or some other brewing software, and you'll know for sure what numbers you should hit.
Regardless, 1.031 seems pretty high for a final gravity to me. If you're going to leave the beer in the fermenter for another couple of weeks, I would definitely consider adding another dose of yeast. My standard "backup" yeast for stalled fermentations or in case I want to add some bottling yeast is Safale-05, since it doesn't add much flavor, is very clean fermenting, and since it's dry, I can keep it on hand for a long time.