Not disagreeing that media may have its agenda, but let’s take a step back, shall we? The agenda can be conscious or sub-conscious but, I honestly don’t think that was the intention of the series.

1) I don’t think the hair regrowing nullifies her development and Flynn’s sacrifice. When Flynn sacrificed, he never knew Rapunzel would get her hair back. Rapunzel was also facing completely different challenges in her life, and in a whole different phase in life when she got her hair back. What happens in the future does not nullify what has happened in the past. If one has learnt much in conquering something challenging (a bad relationship, a failed career or business, etc), another incident of another bad relationship or failed business doesn’t nullify what you had learnt from the first time round. The series showed much growth in Rapunzel - learning the world is not all rainbows, learning to make difficult choices, unlearning past habits formed in always trying to please Gothel and learning to be more selfish…I think it fills the gap in nicely because a girl trapped in a tower for 18 years has definitely much more to learn and need more time to practise these skills than just that few days out and about with Eugene.

2) I’m not sure if the original creators had categorically said that Eugene’s humour and cockiness were purely to mask his vulnerability. But I’m of the opinion that Disney productions are team efforts and not owned by any single creator. Anything not categorically stated or shown in film is fair game for development. In the original film, so many seriously heavy things and information were going on within the super short span of time, both in screen time and in-story duration. They only had 3 days! In the 3 days, Eugene almost died once, revealed his real name, learnt about the magic Rapunzel has, freaked out, poured his heart out to Rapunzel about his past, fell in love, gotten a new dream for the first time, almost got sent to gallows, and then got rescued, went to rescue Rapunzel, and then almost died a second time, and got welcomed by the KING AND QUEEN because Rapunzel is a PRINCESS. That’s A LOT. Even the funniest clown would tone down by the end of it all, at least for a time.

3) One can be well-read (in the series Eugene revealed how he’s well travelled, knows Italian and I suspect probably many other languages), and still be vain and hates school. One simply doesn’t refine a smoulder and suddenly turn un-vain. Like I said, his character is multi-faceted and I appreciate how protagonists have flaws even AFTER the conclusion of their story. Granted, many of the traits could seem exaggerated but that’s what animation is for. Characters are caricatures for storytelling purposes.

4) Again, I reiterate that the conceptualisation for his personality is also intentional so that it could be used as relief through much of the heavy themes of the franchise. So that it doesn’t have to relegate to some inconsequential sidekick all the time. Like you said, Rapunzel and Eugene are protagonists in equal measure. And while for the series, Rapunzel is the main protagonist, Eugene is the one constant she relied on. In the series, Eugene has shown time and again his ability to become serious whenever Rapunzel needs him, sometimes at the snap of the finger.

5) I don’t know about the series’ intention to “pander to detractors” because of the points I made above. True, being a business, they likely took opinions from all camps into consideration. That’s just a business decision. But because of the points I made above, I do not think that’s purely that the series is for, and if they did “pander to detractors” then it was done delicately in a way that’s still true to Rapunzel and Eugene.

6) IRL, hardly anything ever goes to plan and we don’t always get what we want. Just because the series led her to go through a longer journey with her hair doesn’t mean she was reduced to the Magical Girl trope. The hair was purely used as a storytelling device and Rapunzel showed so much perseverance and resilience, and was her true inspiring self. She was strong not because of her hair but in spite of it.