Sounds like you've thought this through pretty good. I just wasn't clear on what conditions you were hiking in; I've seen too many folks who intend to hike only from late spring to early fall by a 15-degree bag "just in case."

Like you, I don't intentionally go out planning to extend a too-warm bag into known cold conditions. (I've got a Megalite for that.) However, I've once or twice had temperatures drop down a little colder than planned, and the addition of jacket and pants made up the difference. So, just for fun, I pushed the Mitylite's limits in my backyard once, using all the down clothing I had.

Now that I've totally exposed my ignorance on your plan, I'll ask one more question: how well do pack animals handle temperatures approaching zero? You wouldn't want to risk losing an animal that couldn't handle the conditions. (And I'll bet you've already thought of this, too.)