After reviewing all of Grindelwald's knowledge, the contract did not cease to exist. No, Edward would not make such a rookie mistake, so the contract will not be invalid until he squeezes the first generation Dark Lord to his last value.
He knew that Grindelwald still had plenty of room to grow--even when he was imprisoned. He did not believe for a second that Grindelwald did not think about magic during his time imprisoned at Nurmengard.
More importantly, Edward wanted the memory of his battle with Dumbledore in 1945. This battle was described as legendary by the people who witnessed it, so Edward wanted to see it and experienced it.
So, he planned to activate the contract again after returning to the present.
After he was done, Edward finally took time to reflect on the brief conversation he had with Death, and all the information he received in just a few sentences.
First, if he messes up with time, he will have to suffer heavy consequences; most likely, Death will be able to break the rules of his non-intervention and deal with him.
Second, there used to be many "Gods" like Death in this world, but after suffering from something called the [Cataclysm], they were either dead or forced to revert to the basic principles that govern realities of this universe.
According to Edward's current theory, he believes that this cataclysmic event occurred about 12,000 years ago.
The reason for that was that Edward had long noticed that this Harry Potter universe is very similar to his previous universe before he transmigrated. The only difference was magic, but the culture, history, and advances in technology were the same with different people.
However, key figures like Einstein, Isaac Newton, Nikolas Tesla, Hitler, and Stalin still exist; people that have impacted the world greatly.
This fact has Edward theorized that the Harry Potter world was possibly just an alternate universe of Earth, but he does not have any proof to back that up.
Back to the point.
One of the theories he once read in his past life was the idea that 12,000 years ago, a highly advanced civilization existed on Earth, but was destroyed due to a cataclysmic event. However, remains of this civilization were discovered by the Mesopotamians, Babylonians, and Egyptians.
Then, they build a powerful civilization and wonders that some scientists in modern time still could not completely understand or explain.
Edward's theory was that similar events might have occurred in this Harry Potter World; that a powerful wizarding civilization existed 12,000 years ago and the Gods like Death even walked the Earth among them.
After going through [The Cataclysm], this civilization was destroyed and the Gods were killed; remains of it were discovered by later wizards, hence the wizards in the past were most likely way more powerful than currently.
Edward believes that magic power used to exist in the environment in the past. But as time passed, the magic power gradually decreased, hence the reason that modern wizards are so weak. This theory also explains why ley line nodes contain magic power, but the environment does not.
The genuine version of this novel can be found on another site. Support the author by reading it there.
This theory can also explain Edward's other theory that the Harry Potter Universe was a High Magical Universe but slowly weakened over time.
Another proof of this far-fetched theory was the Pyramids located all around the world. After connecting them in a certain way, they formed a strange pattern that looked like a magical array to Edward.
After discovering this, he visited all of them during his five years voyage around the world. He discovered that they were located in the most powerful ley line magical nodes on this planet.
On top of that, there are signs of very powerful enchantments in these pyramids. Unfortunately, these enchantments were mostly destroyed due to the passage of time, so Edward could not reconstruct them with the little remaining nor guessed the reason for the Pyramid's construction.
However, now that he has the Gate of World, once he increases its ability to travel back in time, he will visit all these Pyramids again and recreate these enchantments and discover the secrets of the past.
Anyways, he only promised Death not to intervene in the events that occurred in the past, he did not promise not to travel back in time as an observer and recorder of history.
Another plan Edward made after summarizing this incident was to return the Harry Potter Universe to its former glory. And the first step in doing this will be to re-activate these ley line nodes and ensure that magic power is returned to the environment. However, he had a feeling that things would not be as simple as he imagined. Nevertheless, he had plenty of time to realize his plan.
--Scene Break--
After absorbing the essence of Grindelwald's knowledge, Edward placed his attention on Credence; to be precise, on the Obscurus inside of him and his ability to control it.
According to their previous agreement, Edward had to teach him basic magical knowledge, and in exchange, Credence had to allow Edward to study him.
So, during the next six months, Edward did countless tests and experiments on Credence; he even reviewed all his memories of when he first became an Obscurial--an experience which was very unpleasant to the latter.
According to Edward's discovery, an Obscurus is formed when a young wizard consciously or subconsciously suppressed his or her magic power inside the magic core. Then, all the negative emotions of trauma or abuse fused with the magic power creating a dark and powerful parasitic force with tremendous power.
In terms of eastern ideologies, the Obscurus is the negative side or Yin of the magic power inside a wizard. According to his research, the reason that adults cannot become Obscurial is that no matter how dark their thoughts are, they still have some level of control over it, but children do not.
Additionally, after the age of ten, the magic power of most wizards or witches turns into a very stable and hard to control state, hence the reason it becomes more difficult for children after this age to accidentally use different magical abilities based on their emotions. This probability keeps decreasing as they grow older.
Most Obscurial do not leave past the age of ten years old, but Credence was different. The reason for that was his powerful bloodline.
Edward has to admit that the Dumbledore family is full of talented individuals after analyzing his result
Credence's bloodline granted him vastly superior magical powers than ordinary children; thus his Obscurus was way more powerful than an ordinary one.
Secondly, his bloodline granted him a very powerful soul, which then granted him a very powerful mind or spirit.
What Credence did was to separate his mind into two: one was his original self, while the other is all the dark, negative or unhappy memory or trauma that he suffered at the hands of his adopted mother.
This second personality is also the source and origin of the Obscurus. Using his main personality or mind, he was able to control the second personality, the Obscurus.
As a result of this situation, he was able to freely control this dark parasitic force inside of him. Of course, Credence did not have total control as he was easily influenced by the second personality. Once his emotions got the best of him, the second personality would take control, hence the reason that he easily lost control after discovering Percival Graves' betrayal and went on a rampage around the city of New York.
After analyzing Credence, Edward had an idea on how to create the Obscurus Spell, but he still needed some much-needed data--which he will try to acquire after returning to 1992.