Yes.. although excess mortality is the thing that we'll look at in retrospect to gauge the overall human cost of this disease, I don't think it is all that useful for measuring "how well is containment working" in real time. Too many confounding factors (both positive and negative) as you mention.

Personally I think the best metric is one of the simplest -- the raw percentage of positive tests. This is because it simultaneously captures two important factors: if the percentage is high it means that either you are finding the virus all over the place OR it means your testing capacity is so low that you only are testing highly probably cases. If either of these are true, you should not think about reopening.