There's a very simple fact to understand. If we institute a voting system which counts Democrats' preferences among the Republicans, Trump's power over the Republican pols ends.

IRV has not reduced Trump's power in jurisdictions using because IRV does not do so. Sure, IRV let's Democrats rank some Republicans higher than others. But that does not mean that that preference would be counted. As long as there is a candidate rated higher than a voter's highest rated Republican, that voter's preferences among the Republicans are ignored.