Theme direcotry is not ideal for customizing

Indeed all changes to your theme are gone after update, but that also goes for changes to functions.php.
The recommended way is to use a child theme.

Yes, you’re right. But if you have already a well configured theme with a lot of customized widgets, generating a child theme will be a lot of work. And not every WP-User is able to handle child themes.

On the other hand, using e.g. /wp-content/extensions/emefs/… as additional search-path for customizations are adding … two lines of code into emefs.php? ??

Well, in fact creating external files is not the perfect way of doing things, but for now that’s the way the plugin works.
Since I already added the option to give your own template as a parameter to the shortcode, I could add a second option that would allow to specify if the template given is a full path or not, so then the template could go anywhere you like …
Sounds ok to you?

Hmmm … come to think of it, that could pose a security risk. So I’ll need to think of something else …

Thanks for your efforts, but don’t worry too much about this. In the meantime i’m thinking about creating a child-/custom theme because of other circumstances.