The following solution will require a Full Trust form with code, but will allow for a Contact Selector to be placed on a browser-based InfoPath form which will consistently validate, requiring the selection of a user as well as allowing for only one selection.

The code:

public void supervisorEmployeeID_Changed(object sender, XmlEventArgs e)
{
     try
     {
          int supervisorCount = e.Site.SelectChildren(XPathNodeType.Element).Count;

          if (this.Errors.GetErrors("_supervisorCountError").Length > 0)
               this.Errors.Delete("_supervisorCountError");

          if (supervisorCount > 1)
               this.Errors.Add(e.Site, "_supervisorCountError", "You can only select one Manager");
          else if (supervisorCount < 1)
               this.Errors.Add(e.Site, "_supervisorCountError", "You must select a Manager");
     }
     catch { }
}

The above is meant to serve as a reference and should be self-explanatory.

Today when I attempted to completely retract my form (inlcuding manually deleting the content type associated with the form), I was presented with the always helpful error “The content type is in use“.  This is by far one of the more helpful errors you will ever see from SharePoint.  Unfortunately it isn’t clear how to discover where the content type is actually in use.  A quick trip to the content database for the site collection you are working in can show us:

DECLARE @ContentTypeName nvarchar(128)

SET @ContentTypeName='Content Type Name Here'

SELECT w.Title AS [Web Site], w.FullUrl AS [Web Url], al.tp_Title AS [List Title],
	ct2.*
FROM ContentTypes ct1
	JOIN ContentTypes ct2 ON LEFT(ct2.ContentTypeId, Len(ct1.ContentTypeId))=ct1.ContentTypeId
	LEFT OUTER JOIN dbo.ContentTypeUsage ctu ON LEFT(ctu.ContentTypeId, Len(ct2.ContentTypeId)) = ct2.ContentTypeId
	LEFT OUTER JOIN dbo.AllLists al ON ctu.ListId = al.tp_Id AND ctu.WebId=al.tp_WebId
	LEFT OUTER JOIN dbo.Webs w ON al.tp_WebId = w.Id WHERE ct1.ResourceDir=@ContentTypeName

[Credit to Curtis Ruppe]

Once we know where the content type is in use, deleting it through the GUI is a trivial matter.

The moral of the story? When publishing a custom initiation form, be sure to leave the “Alternate” publishing location empty.