After contacting with Ymir in the path, Eren's current power has surpassed "The attack Titan" and "The founding Titan", he has become an omnipotent god-like being. Every details of the whole "Shingeki no kyoujin" history can be seen by Eren, from past to future.

In 130, author showed us some more memory pieces that Eren gained from kissing Historia's hand, most of these scenes seemed reasonable except for two:

1. The scene where the "Young Bertholdt turned back to human after kicking the Wall Maria".
2. The scene where "Falco laid on the battle field looking at the sky”.
These two scenes occurred in a time and place where it is impossible for Eren to be present. Then where did these memory come from?

The Attack Titan can only see the past and future memory from the inheritor's point of view. Therefore, to explain these two absurd memory pieces, there's only two reasons that I can think of.

1. After contacting with Ymir, Eren can indirectly travelled to a specific spot through Ymir's path.This must be a voluntary action from Eren for the first scene. In Year 845 when the young Bertholdt just finished kicking the wall, Dina Titan(who ate Eren's mom) was right next to Bertholdt. However, Dina Titan ignored Bertholdt and walked straight into the wall to eat his mother. I reasonably assume that Eren travelled back to control Dina to allow the history to proceed at the right path. This may explain why the first scene appeared in Eren's memory pieces.

HOWEVER, this explanation is contradicting with Eren's belief that is shown in chapter 130 and 131. As a character who chooses to sacrifice the whole humankind to protect his closest buddies, it would be too contradicting to say Eren sacrificed his own mother first.

This also can't explain the second scene, because Eren has no reasons to travel back to see Falco from a bird's depression angle. Then there's only one reason behind this.

2. Eren becomes omnipotent and know everything that's happening in the world without having to travel to the spot.

This theory sounds ridiculous, but the fact is that Ymir Fritz is connected to every subject of Ymir through the Path. She can change and manipulate their memories, even regenerate and change up their body structure. This shows that Ymir has always been the omnipotent god, but was always under controlled by the king. But now that she is freed by Eren who is her only ally, it is reasonably to assume Eren and Ymir can share their information, power, and memory.

In 131, the non-elidían boy who saw Ymir is a proof that Ymir is always watching every corner of the world. Therefore, under Ymir's help, through the path, Eren's conscious can see any thing that's happening in any time any place for the past 2000 years as long as there are subjected of Ymir present. But the question is

"Does he really need a subject of Ymir to be able to see what's happening there?"

Why in the memory piece, Falco is being seen through a BIRD's point of view.
Why at the end of 131 a BIRD made eye contacts with Armin?
Why did BIRDS repeatedly being empathised in the past two episodes, or even through out the whole story?

Remember when Grisha and Owl discussed about Ymir? Owl said Ymir was a "girl who came in CONTACT WITH THE SOURCE OF ALL ORGANIC MATERIAL".

This reminds me of Zeke. He hasn't been in scenes much recently, but do you remember anyone mentions the power of a Beast Titan? Except for the beast-like appearance, Zeke's Beast power was only powerful because of his royal blood and exceptional throwing skills. In the past, Marley military has always despited Beast Titan and called it weak. But what if-

The Spine Creature, the origin of all organic material, that creates Titan power is actually CONNECTED TO ALL CREATURES. This ability remained in the Ymir Titan at first but was then passed down to the Beast Titan when the Titan power was divided into nine. Yet this power lacks conditions and tutorials for it to be fully used. Therefore when Zeke(Beast Titan) built up connection with ymir in the path, its hidden power of "connecting to all creatures" was triggered and can be put in use.

If Zeke is still capable of this power, controlling all the creatures, then this might be a way that could stop Eren in the future chapters.

This explains the last few scenes of the opening in Season 2 SASAGEYO where all the creatures from ancient to modern all with a heart were presented with Beast Titan. Because they are all connected with the beast Titan through this power.

If this theory is correct, then in ep131, as a omnipotent and know-it-all Eren, he's present at all period of time. The scene "Freedom" with young Eren opening up his arms may also imply that:

1. A long time after Eren clearing out the human world outside the island, he saw the rest of the world that has everything Armin used to talk about it.

2. Eren and his buddies found a way to save the world without killing off everyone.

And since Eren is present at any time and any place and is connected to all creatures, the view of a bird can be passed down to young Eren which allowed him to see the beautiful world with an incredibly high point through the path. His pose, opening up his arm, also suggests the bird. Young Eren tries to let Armin to see this view, as an order, it triggers all three Titan power. The hidden path activated.

The Founding power allows Armin to see some images from young Eren's perspective.

The Attack Titan power triggers to connect young Eren's time period to Adult Armin's time period.

The Beast Titan power triggers and searches for the two birds that are closest to Eren and Armin as the input and output of the images.

Thus, adult Armin was able to meet with young eren and saw the view of Freedom.

Afterall Armin, is always the one that Eren can talk to wholeheartedly.

Hopefully this also explains why in ep1 Eren was crying under the tree, saw short hair Mikasa in the dream, and said "It was such a LONG dream." Because the adult Eren, who has already seen the 2000 years of history and the future tragedy, is connected to the young Eren.

Chapter 131 ends with "The path which diverged will once again...."

....come together.

I initially thought that the beast titan can take up the form a fixed animal of its choice. This is heavily implied in Tom Xaver's conversation with Zeke. So, in Grisha's memories, Zeke is shown playing with his monkey stuffed toy which might be his favourite toy, and thus he chose to take up the monkey form when he became the beast titan. This explains those Dinosaur titans, Whale titans, Owl titans, etc. in the end of the opening song of AoT S2