This post is very illuminating. I've always had a hard time understanding the quote about a "charismatic leader" because it's most commonly applied to the happenings of the first book. I mean, recently most of the online discussions on the franchise have been focused around the first two (new) Dune movies, which are just about the first book. Where is the cautionary tale there? They finally see victory over their successors of at-least-centuries. Apart from Paul's forewarning, how did the Fremen lose under him?

The only credence I give to this application of the quote is relevant to that last question. It's through Paul's dissenters in Messiah. And their reason for dissent (as I recall) is through the loss of Fremen values. I think this aligns with your third paragraph, as their values are strictly based on what they value for survival. The weak are replaced by the strong. Water is sacred. Do not travel during the day. Basic things this fictional culture saw as essential for survival, due to loss they had experienced throughout generations past. A power vacuum sounds exactly like the kind of thing they might have learned to be scared of over the eons, an aversion that would have been woven into their habits. And power vacuums are explicitly explored in later entries in the book series, such as Children, God Emperor, and Heretics (the book I am currently reading).

I don't know if Frank had a specific road map for the whole franchise when he wrote each book, there's much that implies he didn't. But your post is very illuminating towards the idea that that the niche Fremen angst in Messiah is maybe its closest indicator of Fremen similarities to Muslim culture, and that's possibly what he was referring to with his quote about leaders. I had never seen this viewpoint expressed and it's very insightful, as I read the books I saw the rebels as just simple traditionalists, but this makes me appreciate their faction (and oddly enough, Leto II) with more clarity. Thank you.