I've just finished reading chapter 83 and I am still confused about one aspect of the serum. How do we know that it has the ability to cure a gravely wounded person? In this chapter Erwin said that it is possible and many meta writers agree with him but is it an assumption or a fact?
Erwin and Hange are making a lot of assumptions. For one thing, we have no evidence that memories transfer - quite the opposite has been true. They’ve seen Eren struggle to regain his memories and Ymir and Bert are both on record saying they have no access to theirs. On this point at least, I noticed the official translation toned down the language a little.
And there is no canon basis to believe that the serum can restore someone on the brink of death. It simply has not occurred in the narrative. Kenny theorized that the serum would keep him alive, but so far we’ve seen only healthy young people be successfully injected. True, we’ve seen shifters heal from horrific injuries, but they were already shifters when they received those injuries.
It’s possible Hange and Erwin are wrong and we are being set up for a big letdown when it comes to that blasted serum. It’s also possible that Isayama is retrofitting his story slightly, introducing previously unknown abilities into the narrative as a convenience for future developments.