Welcome to the Q&A Forum

This got resolved, so here's what happened for anyone else with this issue. After 6 weeks of the page ranking well on mobile, but not ranking at all on desktop, we: specified the old URL in the sitemap.xml put in an individual 301 from the old URL to the new (ie didn't just rely on the rules we were using for other URLs) ensured only one redirect was required from old to new URL put a link on the homepage, using the targeted keyword phrase, to the new URL (these were in addition to the other technical fixes detailed in the original post above) About a week after this, the desktop ranking returned, with the new URL ranking.  Maybe coincidence, maybe the extra steps were required. But it's worked, whatever the reason.