Here is how the support team explained the priority of what editor template is used in the MVC grid:
"Actually the behavior is if the EditorTemplateName method is not used to specify template then the ForeignKey editor template is used. If the EditorTemplateName method is used - the template which is passed is used. If there is no such template, it fallbacks and uses the UIHint attribute or if there is not such the default one is used."

I'd think the priory should be:
1.)  If EditorTemplateName is specified, use that,
2.)  If UIHint in Model class is specified, use that,
3.)  Then use default editor template (i.e. ForeignKey.cshtml)