But then you've already lapsed into consequentialism, and thus stuck yourself with a mandate to consider the trade-offs between desirable and undesirable consequences.

Yes, and deontologists and virtue ethicists consider trade offs between different principles or virtues.

This is not what deontological and virtue-theoretic politicians actually do.

This is not what consequentialists actually do either. In particular, I've never seen an actual utility function, much less using one to compute trade-offs.

"Look how morally brave I am for being willing to let this sort of thing happen out of pure principle!"

Well, this is also what consequentialists talking about trolley problems sound like.