How much bandwidth does a SD Netflix stream on a Wii use? I'm guessing it might dynamically change quality but it's worth a shot. I haven't really been able to find anything definite on the interwebs.
SD streaming on Netflix seems to pull around 850 MB/Hour. This probably applies to all of the set-top-box Netflix clients including the Wii.
I don't have a good number, as you mentioned it will fluctuate depending on the position within the stream. But what I can tell you is that you need a 1.5 mbps or better connection for it to be "stable" in my experience.
My personal experience is that bandwith usage through the Wii is way higher than through my Laptop (for whatever reason). Maybe the option to dial down picture quality in the settings doesn't get registered through the Wii platform?