Are there any good strategies for conveying the true significance of the results that look obvious post factum?
The best approach may be to address the issue head-on: include a paragraph in the cover letter, in the introduction or discussion sections of a manuscript, and explicitly in the conclusions section of a talk, that sets up a "straw man" critique. Something like: this result may seem obvious because of (expectations in the field; the beguiling simplicity of the final proof; the incorrect assumption that the problem was already solved; etc). But actually that assumption is flawed (and say or show why). Our result is not obvious because (state reasons).