[Escalated to 2nd Tier] wp-admin user language/cookie doesn't work/ignored

Background of the issue:
2 bugs, both have the same cause:

1) use a plugin to modifies wp-admin URL => lang from auth cookie is not used for admin-ajax requests in wp-admin causing any text from admin-ajax requests in admin to be in wrong language

OR

2) Open wp-admin => it will NOT use the language from auth cookie e.g. on "determine_locale" hook
Click any link in wp-admin => suddenly the language from the "auth cookie" is used and it's possibly different from the first request.

This happens because you check the HTTP REFERER to decide on that. See in sitepress->check_if_admin_action_from_referer()

which calls WPML_Locale->locale()
But that same method is used in lots of other places.

The first issue happens bc:
sitepress->check_if_admin_action_from_referer() calls admin_url() internally:

function check_if_admin_action_from_referer() {
$referer = isset( $_SERVER['HTTP_REFERER'] ) ? $_SERVER['HTTP_REFERER'] : '';

return strpos( $referer, strtolower( admin_url() ) ) === 0;
}

However this method gets called immediately when WPML is laoded, but also e.g. when any plugin calls __() initially.

The plugin we're using to modify the admin url has not been loaded at that point (since it's slug starts with a letter after "w" and plugins are loaded alphabetically), so this condition is always false bc our admin url is /staff/ and not /wp-admin/ - but at the point at which you are checking it, admin_url() hasn't been filtered yet, so you get /wp-admin/ and the condition is always false.

Symptoms:
The language from the auth cookie is not used for admin-ajax requests in wp-admin, causing text to be in the wrong language. Additionally, when opening wp-admin, it does not use the language from the auth cookie on the 'determine_locale' hook. Clicking any link in wp-admin suddenly uses the language from the auth cookie, which may differ from the first request.

Questions:
Why is the language from the auth cookie not used for admin-ajax requests?
Why does wp-admin not initially use the language from the auth cookie?