I answered this below, but the game installs on v4.0. The devs forgot to change the version number in game.

If I remember correctly, v3.0 creates a new folder where the save is stored. I think I have it listed in this guide somewhere. Switching between versions should just use their dedicated save locations.

As for the update, I'm not 100% sure. I have auto-updates turned off, but you might not even need to do that if you're manually downpatching.

Not to be even more annoying, but I still have a question:

If I do the downpatch, how do I prevent Steam from updating the game to 4.0? Do I just have to disable updates?

And another thing: you told me that if I do all the endings in version 3.0 I would have the complete gallery of endings. So far, so good. But how do I go back to 4.0 while keeping that? Do I just have to update? Because otherwise I would have to stay on version 3.0 forever.

My saves from version 4.0 don't make any difference, because they only have the endings. I haven't done the ghost list or anything extra yet, exactly because I haven't been able to complete the endings. My intention is: downgrade to 3.0, do all the endings, go back to 4.0 without losing that and complete the other lists, that is, 100% of the game.

Thank you for your tutorial.

I believe I recall seeing that the v4.0 still says v3.xx on the menu for whatever reason. You reminded me of that, and I should probably add that in here.

I already have v3.0 for some reason? Without patching, but I still can't get the ending. I got the candle, chrysanthemum, bear..everything, but it still doesn't trigger the cutscene

I have a full video at the top of the guide! It goes over the steps to not only downpatch, but also a quick walkthrough of the steps I took to get the ending.

Dear friend, thank you very much for the quick response. Yes, it made perfect sense and I expected it to be exactly what you said. It's just a shame that you have to resort to a downpatch, to make an ending, when you could just release a small update, correcting the trigger for the final cutscene. But anyway, let's wait.
Your tutorial is perfect and I completely understood it. I'm just afraid of doing something wrong and losing, especially the version 4.0 save, or it causing a bug when the version comes back.
Would you be able to post screenshots of the tutorial steps, or even record a short video with the steps? I think it would be even more perfect and would make it much easier to see what you have to do. But it's not a criticism, on the contrary. Just a suggestion, really. It was very well explained.
A hug from Brazil.

Glad I could help, dude!

Everything but Kkotnim Sweetheart will still be on your old (4.0) save when you go back, since you're doing that specific ending on a fresh (3.0) save. The only way to have your ending gallery completed is to either do every ending on the downpatch version, or wait for a potential update. Hope that makes sense.

Man, you're a hero. I'll use that guide, for my platinum completion.

If I downpatch the game and return after the achievement, every final that I make, will be there too?