I will give some thought to your main question, because it's one that I do think about a lot.
In the meantime, a quick side-note about mixed-ability grouping: I read in somer research somewhere, years back, that instead of grouping "highs and lows" together, it's better to pair "lows with middles", "middles with middles", and "middles with highs". That way the lower students don't give up so quickly, and the higher students don't get frustrated with them. They work better together when they have at least some common ground. Anyways, I have since used this idea when partnering my students and it works MUCH better! Just thought I would share