I tried creating a similar scenario and it turned out that there was a missing TemplateBinding in the ControTemplate of the TabItem. This may be the reason why you have not been able to get the binding working. The fix will be available for the next SP.
I created a simple project where the HeaderTemplateSelector and ItemTemplateSelector properties are used to select different templtaes for the TabItem. Also the fixed ControlTemplate, so you can use it until the SP.
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