The redirect you're observing with set_session_login only happens with Customer Shop Status set to 1 as you said you've done. That behavior is normal and expected, and has existed for a long time ... (at least since v1.3.0). And the subsequent redirect back to the page the customer came from is also intended behavior.

What you're describing goes further than that where you're specifically trying to intercept and override that normal behavior and instead redirect them to a specific category, rather than taking them back to the page they came from.

The normal behavior of taking them back to the page they came from is a result of the system setting a snapshot of the page they were on before taking them to the login page, and then the login/header_php reads that snapshot and sends them back to that previous page. So apparently your custom code isn't firing before the snapshot redirect fires. ... ie you've put it too late in the code.