I don’t think I quite understand what your scenario is.
Here is how Login Redirect works. It internally uses a WordPress filter called ‘login_redirect’. This filter will be called from wp-login.php after a user has been logged into your system to find the redirect location. Usually it doesn’t matter how the login form is created (by a plugin/theme), it always posts the data to wp-login.php so this filter will be executed.
‘redirect_to’ is a WordPress recognizable parameter, which will be extracted in wp-login.php and passed as an argument to this filter. User Role Editor tries to find the redirect URL based on the configurations and if it finds one, returns that URL from the filter else returns the passed ‘redirect_to’.
Calling this filter and setting its value as ‘redirect_to’ within the login URL will not work, since the filter requires a logged in user to find the URL.