Call to undefined function insert_with_markers

By looking at the source of the latest version, it shows that misc.php only gets included in one place at wp-admin/includes/admin.php. The include there is unconditional. That file in turn gets included seemingly unconditionally in wp-admin/admin.php, and this is required unconditionally as the first line in wp-admin/index.php.

So it appears that it should be the case that's always available in the admin dashboard. If it's not, I'd suggest looking in more detail at what happens in wp-admin/admin.php in case something is going on there which bails out early.

EDIT: Note that PHP gives you the require_once() command which means that it's safe to include the file using require_once more than once, so you don't lose or risk anything going wrong by putting a require_once to that file in your code.

Looks like insert_with_markers() function becomes available during admin_init hook. So in order for your code to work, you should do the following:

function do_my_htaccess_stuff_371705() {
//function `insert_with_markers()` is working now
}

add_action('admin_init', 'do_my_htaccess_stuff_371705');