Another thing you might consider, since adding a mere half inch of serrations may not be all that beneficial, is that you could try the coarse sharpening technique that results in micro-serrations and can enhance slicing ability quite a bit. The details of it are covered in many sharpening articles, especially those written by Joe Talmadge. You can do this on a portion of the blade and still be able to use the entire length like a plain edge as well. It might be worth at least experimenting with it before making some irreversible changes to the blade.
Another solution that may be overkill, but certainly answers the need for both a plain edge and a serrated edge without compromise is to get a two-bladed knife (or carry a second knife). Spyderco's Byrd Wings is a good 2-blade example and there are a few other good ones as well.
All of the serrations I have ever seen are ground only on one side of the blade (chisel ground) and this is superimposed onto the basic V-grind of the plain edge. This has always seemed like a strange geometry to me and does create some sharpening angle problems like you alluded to in your post above. I wonder if, doing it yourself, you could make them symmetrically ground by going at them from both sides of the blade. Just a thought. By the way, suitable round files are often sold as chainsaw sharpening files. And yeah, I think you want any of your grind angles to be more acute than 15 degrees so that you can back bevel it when necessary on the sharpmaker. Certainly more acute than 20 degrees so that you can sharpen it in the usual way.
