The init and plugins_loaded hooks are already run before a plugin is activated. That’s why your first code doesn’t work but the second does.
Regarding your third code: there’s no need to run add_action('myplugin_activate' … inside init. Not everything needs to be hooked to init. Just use
add_action('myplugin_activate', function(){
echo 'myplugin is not allowed.';
die;
});
without any init stuff. Although I probably wouldn’t exactly use die 🙂
Check out the Codex article on register_activation_hook() which has many valuable examples.