Hi David,
Did you publish the entity after changing the display name?
After publishing and attempting an import to the published entity, I don't think there's a need to restart IIS but that's worth a try.
When I've worked with the import wizard in the past, I ran into some problems with picklists and some other issues. The strategy I took to troubleshoot was to create a 1-record CSV file containing only the required attributes. Once I got that working I added on from there. The documentation for the wizard didn't answer a lot of questions we had so that approach helped to incrementally determine where it was failing.
I'm curious as to whether you're using the auto-mapping feature or are using a pre-defined data map?
-Tim