Okay, Guys let me say this now.
There won't be a sequel.
If there will be by some chance, it will be in the future.
The Multiverse Collapsing:
Now, onto why Error didn't destroy while the multiverse was collapsing.
It's because he no longer had the power to.
No the orbs that have been mentioned could not have worked. That was Error's destructive magic so he could have help from the gang. So when his magic left him, so did the magic in the orbs of destruction.
It was stated before that Error was not meant to be a permanent solution. He was meant to tell Ink of the balance, balance out the multiverse, then return to his AU while being freed from Fate's strings.
But when Ink was finally told, his body was already extremely adjusted to how we all know it to be. He wasn't meant to take longer than a RESET's time. But he did. He can still use his strings, he just can't destroy universes. He is now free from Fate's strings though, and now officially Destiny's child all around.
Error's old universe:
In this multiverse, Error's AU was just meant to, like said above, to tell Ink of the balance, be freed from Fate's strings, get his memory back, and return to his universe. What Fate didn't account for was Error turning insane.
Error's universe is the Anti-Void. Everything was meant to just continue on in another dimension without their sans equal until the RESET happened. Without the sans there, the entire universe was destroyed.
The Orb's
The orbs have been mentioned several times. What I see them as is a way to store magic in order to let others use that magic. Such as destructive orbs. Error channeled his destructive magic into the orb to let others just drop it in an AU for it to be destroyed. Another example of an orb is the one mentioned in the second to last chapter. The orb in the tree. Death's magic didn't kill it because the orb was filled with fellow yin magic, making the tree use to yin magic. The way I see it, there are 4 types of orbs.
Normal Orbs - Orbs without magic in them.
Holding orbs - a way to store a monsters magic until later
Healing orbs- whenever someone is holding one, their HP will start to recover.
Magical Orbs- a way to give someone their magic to use. The name will change depending on what magic is stored inside.
The Balance's and Yin and Yang
I've mentions the balance's, yin, and yang a lot so here's description altogether.
The balance of Creation and Destruction is (was) the most unstable and most important. That is because of what happened during the story. If there was not enough room, they universes would touch and fall. If there's too much room, then the universes would not stay up. The yin to this balance is Ink and the yang is Error.
The balance between Life and Death is the most stable and second most important. This is because, without life or death, AU's could not be made. The way I see it is that Fate gave Ink some life powers so he could create AU's. And when Fate formed Error, she gave him some of Death's powers, hence why he is able to touch him. The Yin this balance is Death and the yang is Life.
The balance between Hope and fear comes third because this is what drives monsters and humans to keep going. If a monster has too much Hope (wishes came true) then they won't wish for more. If a monster has to much Fear (Wishes ignored) then they won't try and hope. Error is a perfect example of what happens when there's to much fear in a monster. The yin to this balance is Dream and the Yin is Nightmare.
Finally the balance between Good and Evil. I know some expect this to be like "The Light sanses vs the Dark sanses" but that's not how I view it. I see that the Chara and Frisk equals are the ones in control of this. This balance is the least important because it depends on the other balance's. If the balances are tipped in another way, there will be more Genocide Routes. If the balances are a bit more stable, then more Netrual Routes will take place. If the balances are almost completely stable, more Pacifist Routes will happen. Despite what you believe, I think that the Chara's are the Yang and the Frisk's are the Yin.
The reason the Frisk's are the Yin is that if they never did a Genocide route, they wouldn't have been in a balance. The Chara's are the yang because they are the consequence of the Frisk's actions. Of course, nobody knows this.
Ink
Like I have said to some people in the comments, Ink is not soulless. This is why he feels regret when told. The way I see it is that when a soulless version of Ink uses the vials, he 'feels' that emotion. Just that emotion unless he drank another one. Such as if he drank a yellow vial (Happy) then he couldn't feel anger or determined until he drank the red one.
Let me know if you have a question and I'll update this with your answer.