how important the problem is relative to other problems, what ethical theory to use when deciding whether a policy is good or bad

Apart from those two issues, the other points you bring up are the domain of experts. Unless we are experts ourselves, or have strong relevant information about the biases of experts, the rational thing to do is to defer to expert beliefs. We can widen the uncertainty somewhat (we can confidently expect overconfidence :-), maybe add a very small systematic bias in one direction (to reflect possible social or political biases - the correction has to be very small as our ability to reliably estimate these factors is very poor).

I might still complain about it falling afoul of anti-politics norms, but at least it would help create the impression that the debate was about ideas rather than tribes.

Excessive anti-politics norms are a problem here - because the issue has become tribalised, we're no longer willing to defend the rational position, or we caveat it far too much.