I have watched a few presentations from people who've argued that Uttar Ramayan was not written by Valmiki. Although I don't even know Sanskrit - much less a scholar - some facts that I've tossed in my mind convince me that the Uttar Ramayan is a fabrication of Kalyug authors, & not a Valmiki original, based on a theological reason. The key telltale for me here was the circumstances of Lakshman's passing.
In Balkand, we are told that all the 4 brothers - Rama, Lakshman, Bharat & Shatrughan - were avatars of Vishnu himself courtesy that payash: there was no Lakshman as Sesha-nag, Shatrughan as Sudarshan-chakra or Bharat as Panchajanya (All that seems to originate in Adhyatma Ramayana). In other words, all of them were Vishnu while they were there in life. Now, at the end of Uttarkand, when the devas want VIshnu to return, they send Kal - the god of time & death - to visit Rama, and request him to return. Kal puts a condition that nobody else should see him, and if he did, he should be put to death.
Now, reason Kal would have put this condition was that no mortal should be able to see him, and only reason he's seeing Rama was that he was Vishnu. But so was Lakshman, so Kal wouldn't have had a problem w/ any of the brothers - Lakshman, Bharat or Shatrughan - seeing him. Any of those 4 seeing him still meant that Kal would only have been seen by Vishnu, which would have been fine by him. So Valmiki, having written that all 4 brothers were the avatars of VIshnu, could not have written that Kal would have been angered by Lakshman's appearance.
Another thing that struck me, which somehow seems to have been uncommented on by people: had Valmiki, the same person who described Ayodhya glowingly to a "t", included a chapter on Rama's rule there, then that would have been the chapter called "Ayodhya kand": Kand 2 would have been called something else, maybe "Yuvraj Kand" (since it was about who was to be Dasharath's successor). Why? B'cos he describes Ayodhya as the greatest city ever, & then he praised Shri Rama particularly in Yudhyakand. So had he included a chapter where Shri Rama ruled Ayodhya, that would likely have been the one called "Ayodhyakand".
Aside from this, there is too much of stuff in Uttarkand that makes it untenable as a part of the Valmiki Ramayan. I won't comment on the writing style differences b/w the first 6 kandas and "Uttarkand", since I don't know Sanskrit. Then there are other things: immediately after being crowned, Rama, rather than catching up w/ his family & subjects on what's been going on in Ayodhya for 14 years, sits down to hear the entire histories of Ravan, Vali, Hanuman, et al. Then there is the Shambhuk story where Rama gets the Pushpak, which he had returned to Kubera, to go & find Shambhuk, and also, despite Bharat being the yuvraj, he asks Lakshman to manage Ayodhya while he is out. Also, in kands 1-6, there's not much written about miracles, but in "Uttarkand", there is that weird story about a talking dog who appears in Rama's court and tells him that he was the "Kalapati of Kalanjar" and the circumstances under which he became a dog. Also, in Yudhyakand, Rama decides to give the kingdom to Vibheeshan, rather than massacre even all the rakshashas. In sharp contrast, in Uttar Ramayan, Bharat uses a Kaalastra to wipe out the entire Gandharva race so that his sons can rule over Gandhara. The same Bharat who rejected the kingdom of Ayodhya that Kaikeyi obtained for him from her 2 boons
Some of the other things you mentioned above - be it Lakshman abandoning Sita, Kush-Luv battling their uncles - are not there in the script passed around as "Uttarkand". There, the reason Lakshman takes Sita to Valmiki was that she expressed to Rama the desire to visit some rishis, & while they were there w/ Valmiki, Lakshman explained to Sita Rama's decision to exile her. As for the Ashwamedha yagna, it was Lakshman who led the horse around from kingdom to kingdom, & it went off incident free: even Kush/Luv did not intercept the horse. This story about Kush/Luv intercepting the horse & defeating all of them is a Krittivas original. He's also the poet who had the dhobi story that became so viral
Given all of these inconsistencies, it's safe to conclude that Uttar Ramayan is not a kand that Valmiki wrote, and therefore, it's more likely than not that Sita was never exiled, Lakshman was not disowned & so on. Unfortunately, all the atrocity literature from the likes of Ananda, Bhavabhuti, Kalidasa, Krittivas & others left people w/ a very distorted view of Shri Rama
This is quite thorough explanation
Tbh I haven't yet read whole Valmiki Ramayan but I have heard it many times in Ramayan Path
The characteristics of each character not just Shree Ram is very different in first 6 kaands and uttar kaand. In yuddhkand ending Valmiki ji wrote a very happily ever after ending, don't know how it got upside down in uttar
kaand. The most logical explanation would be they're written by different authors, because uttar kaand seems more of a dwapar or Kalyug story.
Pyaar Kii Ye Ek Kahaani
Kyunki Saas Bhi Kabhi Bahu Thi 2
