https://www.theguardian.com/science/blog/2016/dec/05/ramesses-ii-victor-of-kadesh-a-kindred-spirit-of-trump
Leads with a possibly unnecessary comparison with a modern political spinmeister: it might be better to keep that aspect out of this discussion. But, although the article won't tell our experts anything new, it's a good summary of the evidence and the way that new finds change our knowledge.
The first person to bring all the extant evidence together (apart from the enquiry from Hattusilis why Ramses treated the battle as a victory) was Immanuel Velikovsky in Ramses II And His Time. The actual scale of Ramses' defeat was quite staggering.
As to why despite the seriousness of the Egyptian defeat there was no treaty for another fifteen years, a significant event four years after the battle and various campaigns in Asia Minor provide explanation.