I have a request of adding a "Please explain further, if necessary" option to all my survey's questions. I have tried using "Other" but this is only available to specific answer types. Do I miss something? Is there some kind of workaround?

No, text inputs are not available in all question types. In some cases, you will need to add a following short or long text question.

You mean something like this, wehre you could add additional questions (like one/multiple line textfields) behind every question?

Featurerrequest on this topic:
bugs.limesurvey.org/view.php?id=7367

And since the URL limit restriction seems counting attachments inserts as URLs when a URL is posted I have to add the second feature request link in separate post. Nicely done. Best restriction ever.

bugs.limesurvey.org/view.php?id=11988