RhinoFish - 4:25 pm on Feb 21, 2013 (gmt 0)
Very complex question you've asked there... :-)
If your tracking is solid, test, see what works.
In the situation you've described, the loosest I'd go is Modified Broad match (but I'd use Phrase and Exact and let them be low volume if they turn out that way).
Mine for hundreds, if not thousands of negative keywords.
Dig into Analytics as if you're life depended on it (my livelihood does).