|"Tried to install from Intel again per your link/ suggestions, mickliq. As before, after downloading, got a notice saying driver isn't validated for this computer, and to check in (again) with Dell."|
You probably have to un-install the Dell video drivers BEFORE you install the Intel video drivers.
"Monitor requires 1920 X 1080 resolution, max available for second monitor on the Dell is 1400 X 910. Works, but res/ proportions are obviously wrong."
For the time being, pick the highest resolution that has the same ratio of width to height, if you can, when you divide the width by the height,
1920:1080 - 16:9 - 1.7777 to 1
and turn on Clear Type - makes type/fonts on LCD screens look clearer.
(1400 X 910 - 1.5385 to 1)