Madoka died stopping an Eldritch Abomination three times, in one of those times she was about to die and SHE would become an Eldritch Abomination herself but decided to save her friend's soul AND then asked to be killed rather than bring more suffering to the world.

The fourth time, she was completely powerless, she didn't have any kind of magical training, she was just a regular little girl.

She utterly destroyed the Eldritch Abomination in a single hit.

The downside of this causes her to become this goddess of destruction that would have consumed the entire world in a matter of days, not even a week. She would probably go looking to destroy any life in the universe sooner than later if the time loop hadn't happened.

"NOW" she knows of all the danger there is, she knows she can become this incredibly powerful monster, she knows she would give up her life and soul and she IS ready to still do it.

How one would expect her to be weak is beyond me.

I doubt she would be a threat to anything other than earth, otherwise QB's plan is for nothing. I'd imagine that once the Earth is either killed off or enough people are killed, she will die since witches need grief to live, so if there are no humans to feel grief, there is nothing to sustain her so she would die long before she became a threat to anything else. This is assuming that QB is telling the truth, which I believe he is since even though he has withheld information, he has not, to my knowledge, actually lied about anything.

Who's to say that Earth is the first place this has happened? Where exactly did Walpurgis come from? Is it the Witch!Madoka of another world? After all, Kyuubee said the Incubators took interest in Earth. He never explicitly stated they never cared about any other worlds. A quota suggests that more than one world is involved in their plans. It would fit in perfectly with his habit of never saying more than necessary. And it would explain where the hell this space witch originated.

Unless I missed something which renders all that irrelevant, which I often do, in which case y'all can ignore that.

Earth is likely the only place to have this happen since he cites human emotions. At any rate, once QB is done with Earth, there would be need to interact with other planets. He showed absolute disinterest in humanity, so I'd imagine he would be just as disinterested in everything else.

Also I still don't know how Homura learned about Incubator, I don't recall QB actually stating his purpose in the previous time loops

Yeah... no. Witches don't eat people to survive. They eat people to spawn new witches from familiars. The ones who need food to grow are the familiars, to become new bodies for the Witch.

As somebody already mentioned, the fact there's a Quota, and that destroying Earth doesn't cause any kind of inconvenience for the energy harvesting, indicates that Earth is just one of many planets, not really important in the bigger picture.

Madoka is an anomaly, as implied by Kyubey, and so will be Gretchen, probably in more ways than one. Her witch card has been released a while ago, read it. Also there's no implication whatsoever as to whether witches need anything at all to stay "alive" - in fact, it would deny Gretchen's purpose itself (confine all of humanity in the "dream world" - her witch maze, effectively eliminating all grief).

Earth isn't important, humans, specifically human emotion is. I thought QB said that humans were unique in that our emotions produces more energy than it takes to create it, basically we are a perpetual motion machine if we can be harnassed. That said, once Madoka transforms that is the only energy that will be needed, then mankind will be ignored and so will, presumabely, the rest of the universe. I don't see enough evidence that Madoka is a threat to anything other than earth since if that were true, QB's motivation is flawed since he exchanges one doomsday scenario for another. Unless the next two episodes reveal that hes just a gigantic dick, which will invalidate the struggle up until this point.