Lua, why in the world do you have a post-war verse if Snape dies? How does he survive Nagini and how in hell does he become 20 again?! Well, young grasshopper, let me explain...
Firstly, let's talk about Nagini ━ Voldemort's danger noodle, and cause of Snape's death. Ignoring the fact that the movies made her a boa constrictor ( there is no evidence that she is a boa constrictor, but to my knowledge venomous snakes bite once and wait for the venom to take effect, and movie Nagini just kept going. which is not boa behaviour nor viper behaviour so, basically joanne really said snakes and did no research, huh? ), Nagini is a magical snake...which I assume ( and you should too ) means magical venom.
The venom here is the main factor of all of what will happen. So, there are three types of snake venom: Cytotoxins, Neurotoxins and Hemotoxins. Cytotoxins cause complete cell destruction within tissue or organs, which is called necrosis. This toxin helps digest a snake's prey before it is even eaten. Neurotoxins affect the nervous system, they mess up with chemical signals sent between neurons, this can cause muscle paralysis, which in turn impairs with breathing and leads to death. And finally, Hemotoxins affect a person's blood, more specifically the blood coagulation process. Hemotoxins can have two effects, either they cause massive internal bleeding or they can cause platelets and other blood cells to clump together, which makes huge clots and stops blood flow and can lead to heart failure.
From that, we can definitely infer that Nagini is a Hemotoxic snake of the first variety, given the amount of blood that was smeared all over the shrieking shack. The blood though, didn't come from Nagini continuously striking him. How so? Well, Haemostatic effects caused by snake venom manifest through local and systemic haemorrhage. Meaning, the moment the venom began to flow through his veins, Severus was bleeding from every organ and wound on his body, including but not reserved to, gums, recently healed wounds, the bite, internal bleeding. Everything bleeds. ( please note that these are the results of a normal, shall I say, "muggle" snake bite. Nagini is a Magical Creature with Magical venom so just...amp that up 150+%. )
So, covered in his own blood and with magic snake venom running through his veins destroying everything, how did he survive? Well, Magic, blood and snake venom, obviously.
It is unknown what the effects of Nagini's venom truly is, though if we look to the human snakes and venomous creatures, such as bees, their venom has been used for cosmetic purposes, specifically in the anti-aging sector. Research shows that snake venom temporarily inhibits muscle activity, and that can prevent and reduce fine lines and wrinkles, or signs of age. Bee venom forces the skin to regenerate by causing an inflammation, which makes the body produce collagen and makes skin look rejuvenated and radiant. Essentially, venom is used in these products to trick the body into producing that which it stops after a few years, such collagen, by introducing chemicals that trigger the "inflammation alarm", and through the blood it carries on with the process of regeneration.
These products and effects in the human world are small, they aren't miracles, but Nagini is not a normal "muggle" snake, she is a magical creature so I assume, the effects of her venom would be a lot more impacting.
Blood itself has been known to be around lore of rejuvenation. The speculation about Elizabeth Báthory and her killings, whether they were simply done by a sadistic woman, or perhaps a ritual to try and stay young forever as she allegedly bathed in the blood of young girls. The whole concept of vampirism, alludes to blood being the elixir of youth, or eternal life. And recent studies show that perhaps, age reversal is possible with diluted blood plasma which you can read about ( HERE ) but basically, the concept is blood over time starts getting saturated with bad stuff, and so what if we wash that way, with something neutral? The tests in Mice were very promising.
Severus has always been different, his accidental magic as a child manifested much earlier than normal, and not in the usual way his mother was used to seeing. He didn't just do magic when he was scared, or hurt; his magic was just so present in him that it manifested while he was happy, or just walking around the house. Like it was a living entity within him. The reason he is so fidgety when he is under pressure is because he can feel the magic kind of flowing within him, not only that but his sensitivity to magic itself is one of the reasons he hates when people use magic while brewing potions, because he can feel it interfering with the final result.
During that situation, his magic was all over the place ; self preservation screaming "run" but part of him didn't want to run, part of him just wanted it all to end, though you can hardly control these impulses when you are scared and nervous, so his magic just reacted. And it reacted not only to Nagini's first attack, but to her venom in his blood and to the very fluid surrounding him. This combination between Nagini's magical venom and Severus' own magic cause the effects of the venom to change. So instead of destruction, of completely damaging his organs and tissues, the venom started to slowly regenerate all of his cells in a deep and intense level.
So intense in fact, that it completely de-aged him to his 20-year-old body and mentality. When his body was found, he was already 20 years younger, but he was not awake and he was barely, barely breathing. It took him months to wake up ; his magic and Nagini's venom still clashing within his body. And that is essentially how he ended up the way he did after the war.