I have two days to thaw a 16 pound turkey; Can I start thawing it in cold water, and do the rest in the fridge?
It's fine to thaw for a few (or several) hours under cold water and then put it in the refrigerator. Just keep the water cold and refrigerate immediately upon removing the turkey from the water. The USDA and Butterball are being extremely overly conservative, most likely because a lot of people are really bad at following instructions. Cold water good - warm water not good

As a matter of fact, defrosting under slowly running cold water for several hours now, and refrigerating until time to cook is exactly what I would do.

I happen to live in a part of the world where I can be guilt-free about slowly running cold water for hours. There is no such thing as a water shortage here, the melting glaciers are extraordinarily huge. Thawing a turkey that way in some parts of the world (and even here in the US) would be an environmentally unsound way to do it. So in those parts of the world, be sure to buy frozen turkeys several days in advance so there is plenty of time to defrost in the refrigerator.