Here is what ive found after researching online and making backup save files to see what triggers different ending: The most important trigger is when Owl ask u to betray Kuro; if u betray kuro u get Shura ending (and two unique bosses only for this ending, very cool). if u dont then u can pursue the other 3 endings. Immortal Severance is the normal ending, requires no extra work, simply complete the game after u defeat Owl. Purification ending requires a series of eavesdropping on important npc: 1. Eavesdrop on Isshin and Emma after defeating Owl (u can do this b4 u trigger Owl boss fight too) 2. Respawn area and go to Kuro's room, go behind the statue in the room and hug the cardboard wall to eavesdrop on Kuro. 3. Talk to Emma about Kuro (respawn area if no new dialogue is available) She could move up the stairs near Owl boss fight. She will ask for sometime for research. 4. Respawn area again to talk to Emma, and learn that shes going to visit a grave 5. Talk to Emma near "Old Grave" idol and learn she will go talk to sculptor 6. Go to Dilapidated Temple and talk to Emma and sculptor. After the dialogue with sculptor he would suggest u to eavesdrop on his dialogue with Emma. Go around the temple to find a hole to eavesdrop. 7. Talk to Emma after eavesdropping and she will give u a second bell. Pray again at the buddha to go back to Hirata Estate 8. After two mini boss u will fight a very hard boss. After the boss u will get key item for Purification ending. Last ending in next comment.

lol

Dragon's Homecoming ending requires interaction with npcs, especially the Divine Child of Rejuvenating Water (and there i was on my first playthru wondering why she was so unimportant in the story). And its rather complex imo, so this might be a bit messy:
1. The first item u need is Holy Tome: Infested. If u finished up Senpou temple before Genochiro, by the time u reach the "Main Hall" idol, u will see a monk praying to buddha. Talk to him to get "Holy Tome: Infested". If u go to senpou temple after Genochiro, you wont see the monk near the idol, instead u do the monkey boss fight there. In this case to find the "Holy Tome: Infested", dive down the pond near "Temple Ground" to find said tome.
2. The second step is to interact with Divine Child of Rejuvenating Water (U need to beat Genochiro to do the screen folding monkey so that u can get to her in inner sactum) She will give u 3 rice the first time u meet her. Eat it and refresh area, ask for more, she gives u one. Eat it. Your goal is to get her to start snoozing and it seems like the trigger is eating her rice and giving her the tome. Anyway once she started snoozing b4 u talk to her u are on path for this ending.
3. Unsure if necessary but kuro will give u sweet rice ball if u were damaged after reaching foutainhead palace. Eat it.
4. After asking for more rice from the Divine Child of Rejuvenating Water. She would appear to be sick, if this doesn't happen u either didn't give her the tome or u didn't eat Kuro's rice ball (unsure about this one) She will ask for Persimmons, can be found as loot around temple or buy from memorial mob merchant around "Shungendo" idol. U can also get one free Persimmon from finishing the crying giant's quest at the start of senpou temple.
5. Once she becomes better after eating the Persimmons, ask for more rice. She will give u Rice for Kuro, (if this doesn't happen then eat the rice she gave u and refresh area) Give Kuro's rice to Kuro and he will make another rice ball. Eat it, Sekiro will mention it taste super good.(This entire 5th step could be unnecssary)
6. Anyway the goal is to get her to move to the halls of illusions where u caught the monkeys, so keep asking for rice (eat it so she gives u more) and do all the steps if she won't move. Once she moved to halls of illusions talk to her to learn u need one more tome.
7. Find said tome in the cave around main hall idol in senpou temple with creepy crawling enemies, give tome to her she will mention u need two serpentine fruits next.
8. From the first idol at senpou temple, drop down near the crying giant and the monk praying at the broken bridge u will a one of those short guy with the big hat. Backstab him and use the puppeteer ninjutsu, he will operate the kite that is near him.
9. Continue on the temple level until u reach a old lady at a cliff, cilmb the big tree near her to use the kite to jump across the valley. Look down right after u reach the platform to see a series of jumps, do that and u will reach an idol (Sunken Valley Passage? Cant remember name) From the idol just go forward a bit and jump down to kill the Serpent, (it wont be there if u havent trigger the event with it in sunken valley).Fresh Serpentine Fruit acquired.
10. At the bottom level of sunken valley there is a cave with lizards and traps (go back from the idol b4 guardian ape fight for faster access, u can also see the old lady at that idol) Continue down the cave to encounter another serpent. Drop down on your left where some spirit emblem is, u will see a monkey cowering on the ground. Puppetteer and he will draw the serpent away, allowing u to get pass the serpent and acquire Dried Serpentine Fruit. U would need to teleport away since the serpent block your way out.
11. And there u go, give the two fruit to the Divine Child of Rejuvenating Water and refresh area a couple of times, she will give u "Frozen Tears" and now u can choose the "Return" ending at the end.

It would appear step 3 is necessary now.

UPDATE: all unnecessary steps and unsure steps should be done in order to avoid bugging questline.

I updated the wiki entry with the "Return" guide from the comments. The third step is definitely unneccessary, but I am not sure about the fifth, which is basiclally the same as the third.

Updated the Shura Ending, sorry about the weird font and background, don't know what happened.

For the "Purification" ending do I need to kill Lady Butterfly before I go to the Estate with Emma's whistle to kill Owl there?

Why does going to Hirata Estate for the 2nd time lock you out of 'Immortal Severance ending'? Can you choose to give only the Dragon tears to Kuro at the end, ignoring the Everblossom you got from Hirata Estate and still get the Immortal Severance ending?