HEllo,
Two questions:
1. Is there something else I need to do to tell it to use single-wpos_portfolio.php instead of single.php
>>No need to do anything else, this is the correct way, it should work.
2. Why do I not have the “Sidebar and Header Attributes” selection box in the portfolio admin area, like I do for regular posts? Is there a way to add this? If I had this option, I could simply select a different sidebar, and that would be sufficient if I can’t get the full width to work.
>>This depends on your theme
It looks like you are using PRO plugin please find our site and raise your issue there, our team will help you, this is free plugin form.
Thanks