The only negative comment I would make about your question is that you quote a sentence without giving the context.
C'est toujours la même chose. La nourriture est toujours détestable, et les prisonniers sont toujours innocents. Ce qu'ils désirent, c'est toujours la liberté. En avez-vous d'autres ?
In this sentence en is without doubt a personal pronoun but there is not enough information to tell if it refers to nourriture or prisonners. @StéphaneGimenez pointed that out to you in a generic sentence :
Please give proper attribution to any book excerpt or quote.
where you understood "proper attribution" only in the legal sense (publication), whereas it should have included the title of the actual story (Le Comte de Monte Cristo) as well as enough sentences that come before in order to give context. Since the original text is freely available on the internet @jlliagre could find it (note that you have a different simplified text in your graded reader).
how I could/should improve the linked question (both the title and the body of the question)
My advice : Give exact and sufficient context where you have read / heard the words you want help on.
Apart from that your question is very clear.
In the last sentence, how should I understand "en"?
Was answered by @Leo and @jlliagre after he looked for the context you did not give.
What is its function in grammar?
Was only partially answered in @Alexandred'Entraigues's deleted answer.
Doesn't "Avez-vous d'autres" mean "Do you have something else?"
Hasn't been answered yet.
You could have asked in comments for precisions about the unanswered parts of your question but these unanswered parts are clear enough in your original post.