I repeated the exercise under Win10, and to my chagrin the same errors were back again. After some amount of fiddling around in the IE11 debugger, I noticed that it seemed to default to rendering as IE7, and the debugger even gave a message saying that this was because the Compatibility Setting "Display intranet sites in Compatibility View" was set. After I turned this off, all was well - again, no more JS errors. So IE configuration settings affect how the renderer works, and a local file loaded with LoadPage is counted as such a "site".
This setting appears to exist because larger MS customers with intranets where they run old unmaintained websites need to have IE11 run in quirks mode.
Is there any way to affect IE11 settings from within a Xojo app?