I appreciate this analysis, and I think what you’ve described will be a second-order effect of the photos. Feminists (myself included) need to guard against being tempted to attack this woman for political purposes, for exactly the reasons you’ve outlined: it would be hypocritical, un-feminist, and would shallowly place political gain ahead of genuine principle.
I think, however, the larger effect of these photos will be to further alienate Christian voters and cause them to stay home (and by Christian voters, I mean voters who put their Christian values first, and not the “cultural evangelicals” who claim Christianity but practice something else). If there were any “traditional values” Christians still planning to vote Trump, they’re staying home now.
As for the Post’s motives, I think it’s just the clicks. The Post may be in the Trump camp, but more than anything it’s in the “whatever sells” camp. And nothing is going to generate more clicks than a naked model. It is a tabloid, after all.