Soooo, after I finished the "Destroy the Dark Brotherhood!" quest, I decided to make it unhappen somehow. No saves, only console.
I managed to revive all of them and toggle them to "friendly mode" as they first were to attack me.

The only thing is ... Astrid won't give me any quests and won't go to the "main hall" (with the water) to welcome me (?).
After I told everyone of the DB who I was and asked them who they were, the redwardon said to me before I could talk with him something like that:
"Go down there, everyone is waiting!". Now they all went down there, waiting for Astrid, but she won't come and I don't have something like a "Welcoming Quest".
Is there a way to remove the "Destroy DB!" from my accomplished quest lines? Or could I get Astrid to act normally without removing it, though it would be sorta bug? There has to be a way. Q_Q!

There is a way actually, start a new character.

It's the divergence of the quest line that has made all of the DB quests unavailable to you now. Once you killed Astrid to kick off "Destroy the Dark Brotherhood", the game made any quests for the regular quest line completely unavailable.

Even though you have them all resurrected, since your game still sees you having completed the Kill quests, it won't make the regular quests available.

Only thing I can think of is to start a new character.

you cant have your cake and eat it to, once you decided to kill Astrid you walked down a road that there was no turning back from (aside from loading a previous save)

But isn't there any way to delete the kill quest out of my gamelogs?
"resetquest DBDestroy" only changes the quest from "already finished" to "active", while "resetquest db02" (which is the quest in which I have to kill one of the three guys) does the same... there is written, that I killed Astrid already, so it isn't a "clear" reset if you get what I mean. There just has to be a possibility and there isn't anything that followed the "Destroy DB!" Quest.. ><

Then that should obviously tell you that there is no way to do both quests. The game is apparently designed to keep people from doing exactly what you are trying to do.

Why are you trying to cheat this out any way? Doesn't it make sense that if you kill the DB then you can no longer get quests from them? They're dead, that makes it kind of hard for them to give out quests.

Just load from a new save or start with a new character. It's your only option.

Well yeah, I realize that I will have to do a new character (because I don't have an old save..) Q_Q

Learn to take responsibly of your actions. That is the only true cure for this. In the future, perhaps, you can use the CK to resolve this issue but the safe bet is just to start a new character and watch where you swing that thing

I understand what you are trying to say but I think its bigger than that "resetquest db02" is only a small little quest within the DB quest line where as the Destroy the DB is is own quest line that is triggered by the act of "killing Astrid" whish is outside of that quest line. So to reset everything I think you need to look how to get the game to look past you killing Astrid and I am not sure you can as once the game see's a quest NPC as "dead" it treats them as "dead"
If you do find the console command to reset it I would be interested to see how you did it.

Sorry, you have to start again.