Pronunciation of PLUS in Superlatives

Based on:
- The article from lawless french
- The following stack exchange post: https://french.stackexchange.com/a/275
- The following article: https://www.thoughtco.com/how-to-pronounce-plus-french-adverb-4084872

I would say:

In your examples "plus" is still either a comparative or superlative adverb and therefore pronounced without an "s", since there is also no potential for confusion with the negative particle "plus" (no more) and it is also not at the end of a sentence.

On a practical note:
I would say it is more dangerous to omit the "s" erroneously than to pronounce it erroneously.
EDIT: In case you want to express "more" with "plus" that is.

Hey, but in my 4th example of Comparative -  J'ai ramassé plus de feuilles que toi. --> 'Plus' here means 'more' !

And as per the Lawless French article, quoting -  "Plus de followed by a noun can be affirmative ("more") or negative ("no more"); When plus means "more," it’s pronounce [plys],* and when it means "no more," it’s [ply]."   https://www.lawlessfrench.com/grammar/plus/

So in the 4th example of Comparatives with Nouns, the pronunciation should be (plooS) instead right?

Even in this lesson on Comparatives with Nouns Plus de ... que/Moins de ... que/Autant de ... que = More ... than/Less ... than/As much/many ... as (Comparisons with Nouns in French) - in all examples, it is pronounced as  (plooS).

Therefore, in Superlatives with Nouns and Verbs (the 1st 3 examples) the pronunciation should be (ploo) and only in 4th example of Comparatives with Nouns pronunciation is (plooS) ?

The text you quote from Lawless French refers to "Plus" at the start of the sentence, so it is not really applicable to your 4th sentence. However, you are totally right that in the Kwiziq lessons on "plus de ... que" the "s" is pronounced even in sentences where there can be no confusion with the "plus" particle.

Also, in a post by Aurelie on this topic ( https://progress.lawlessfrench.com/questions/view/when-do-i-pronounce-the-s-in-plus ) the following sentence is given as an example when the "s" in plus must be pronounced:

"J'ai pluS de billes que toi."

This is in contradiction with the Stack Exchange article and also with the other referenced article ( https://www.thoughtco.com/how-to-pronounce-plus-french-adverb-4084872 ). But maybe they are just wrong.

There also seems to be the option to drop the "s" for euphonic reasons brought up in the same post by Aurelie: "Il a plus de mille livres."

My sense for euphony does not allow me to get the difference between the two sentences. Both sound fine to me with or without "s".

With all the conflicting information on the internet, I think personally I will stick with the rules given in the following document by a French university teacher:

https://people.unica.it/maudcharpentier/files/2020/11/Plus-plyz-ou-ply.pdf

According to these rules all your example would be pronounced with an "s".

Aurelie's euphonic example is also covered under "Plus avec un nombre".

After I had noticed that the document, which I had linked in my last post, contained errors in the phonetic transcription of "plus", I started to google again and I think I have now found a nice table on wordreference.com that covers most relevant cases of the pronunciation of the "s" in "plus":

https://forum.wordreference.com/threads/plus-prononciation.444986/

1. Je travaille le plus dans ma famille.

Both variants are possible but the variant without "s" is less often used and more formal register.

2. J'ai le plus d'amis.

Both variants are possible. The variant without "s" is more formal register.

3. J'ai ramassé le plus de feuilles.

Same as 2.

4. J'ai ramassé plus de feuilles que toi.

Only the variant with "s" is possible.

Note that in "plus que" without a noun e.g. "Il mange plus que moi." both variants are possible. The variant without "s" being formal register.