EXPLAINING VIKTOR'S "only you can show me this" DIALOGUE, and WHY VIKTOR ISNT THE MAGE
(In a way that tries to get rid of the inconsistenties that come with what canon showed us)
First of all: Viktor isnt the mage, Jayce just thinks Viktor is the mage due to him wearing the cape and standing with a staff. The mage clearly has different hands and markings on the outside of them - something Viktor doesn't have, and which is too prominent of a design to forget or ignore. I truly believe that, it Viktor were the mage and they knew this beforehand, they would a) make the mage in s1 look more androgynous and vague, or b) make the Mage Viktor we see in s2e7 look more like the mage from s1 - tattoos included!
I also would hope that they would show at least one scene of Mage Viktor using the arcane like he supposedly did in season 1.
I believe the mage, whoever he is, saves Jayce in every universe, but the only thing that differs is which stone he grants him. This is I guess the "node" from which different timelines diverge.
In one timeline, the one Ekko goes to, Jayce gets stone "x", which is different than the ruin he got in our universe. Hextech isn't invented due to the job gone wrong (perhaps the stone was too volatile and killed Vi) > Jayce and Viktor don't meet > Viktor isn't consumed by the Hexcore > Viktor doesn't turn Arcane Evil (so there is no need to show Viktor "this", aka what hextech/the arcane does to the world, as nothing happened in this world).
The one Jayce goes to, the there-Jayce clearly failed to stop Viktor on top of the building. As we don't see any mannequins there, I think this must also mean other things went differently: here Jayce received ruin "y", again not the acceleration ruin. Without this, Ekko cannot create the Z-drive > Ekko didnt come up there > didn't jumpscare Viktor > Jayce couldn't change Viktors mind > everything went arcane > Arcane Viktor is the only one that survives, leaving the world full of empty shells (the ones he tries to convert in our universe) and the mannequins he made for the attack.
This is also why our Jayce, when at the top and sitting with his hammer in front of him, gets "Scared". He recognizes the position he is in, and that this is where it went wrong in the other dimension. This neatly ties in with both the apparent different universes AND timelines. Both the ruined Piltover Jayce sees and our Piltover are the same universe - but one shows a timeline where Viktor wins, and ours becomes one where Viktor is saved.
When Viktor says "only you can show me this", he doesnt mean "only Jayce can show me this", because we have seen that Jayce, in other universes, did not manage to do so. Instead, he means "only you - as in - only THIS Jayce in particular can show me this". Only OUR Jayce. We do not know what went different in all universes, but ours is, despite all its tragedies and horrors, still the only one wherein our Viktor can be saved after he gets corrupted, and can be saved by our Jayce alone.
And I think this moment is not only the moment Viktor sees what the hexcore/arcane can do to the world, but this is also the moment that he realises that the "affection" between him and Jayce is special. He is suddenly and harshly confronted by what he had, and by what he is about to give up were he to continue.
And, dare i be a bit bold: it is because this Jayce and this Viktor fell in love. I think it is a very deliberate choice to have the shot where Viktor looks up at Jayce after meeting, with an inquisitive, curious (and flirty) look. It is this moment, this meeting, that changed their lives. That made this universe and this timeline different than all the others.
Only you can show me this. Only you, because you love me, and the me that you know, loves you in return