AND SO: there is no need for Intel to apologize, because the inconvenience was caused by Dell. They sold me an adapter that DOES NOT REALLY support their own U2711 monitor!!!
At the same time that I contacted you via this community, I was becoming suspicious about running an HDMI cable to the U2711, because the monitor itself had been delivered with the DVI-D dual link cable that I had previously been using, even though it has an HDMI input. Searching the web about the U2711 then revealed a bunch of conflicting information, but this included claims that the HDMI port would support only 1080p (e.g. on page 33 of the user manual!). (A few folk out there seem to have accomplished complex workarounds on the HDMI limitation, getting higher res.)
Dell does not sell a USB Type-C (Thunderbolt 3) to DVI-D adaptor, so I searched and found the Cable Matters #201019 Adapter that does just this, getting 24 hr delivery.
AND VOILA! 2560 x 1440 resolution. Your Graphics 620 instantly responded, no adjustments or fiddling needed.
Thanks for having responded to my plight so quickly and seriously. Now onto getting Dell to accept a return of their useless adapter.