note that you have to look at the net effect after taking into account effects in both directions; I would still put substantial probability on this estimate ending up effectively negative, also.

I agree and should have been more explicit in taking this into account. However, note that if one assigns a 2:1 odds ratio for (0.1%-1% decrease in x-risk)/(same size increase in x-risk) then the expected value of preventing nuclear war doesn't drop below 1/3 of what it would be if there wasn't the possibility of nuclear war increasing x-risk: still on the same rough order of magnitude.