I wonder if you don't mix-up "captive portal authentication sent to proxy" and "squid support for radius based authentication". Its no clear to me what you really want to achieve.
Why don't you, for testing purpose and if possible, stop using captive portal and focus on "Squid + radius" authentication?
For this, Squid wiki will still help.
Once this works, you can go back to the initial config stacking Captive portal and Squid.
This aside, I've to admit that I don't really understand what's the added value of having stack of authenticated captive portal plus authenticated HTTP proxy.
What would the first one bring to the other? Is there something I miss here ?