First thing. Ekko is a timetraveler of course tiem travel was going to be involved even since S1 referenced the Z Drive and reinforced by this scene
I don't mean Ekko, although there are some questions to ep7, it was probably the best of the entire season.
I mean future!Viktor being the person responsible for saving kid-Jayce and his mom in the past, inspiring Jayce to invent Hextech, sending the entire story into a classic "chicken or egg" tumble, not to mention him pontificating about muh 16 million timelines.
Jayce wants to stop Viktor so he goes to stop Viktor by all means he has at his disposal
Jayce actually suffers the least inconsistency out of the main cast IMO, but he also doesn't gain much. His decision to save Viktor using Hexcore is entirely in-character, so is his horror over discovering Hextech's potential for danger, and then his resolve to stop Viktor from realising it. What does suffer is his dialogue in Act3, the speeches he gives are really rather cliche - not to mention everyone just automatically accepting him back onto the Council after months of disappearance.
The Black Rose Mel's "magic"
The entire storyline of Mel in S2 is garbage. In S1 she's an exceptional supporting character for Jayce's arc, starting off as a devil on his shoulder and slowly changing into an angel as she develops feelings for him and external circumstances make her question her morals.
In S2 she's just...there. She doesn't impact anyone's story, she barely interacts even with Jayce and Ambessa, let alone other characters, spends a third of her screentime in captivity, breaks herself out and spends another third being Golden Magic Jesus. Like, her saving Caitlyn in Ep9 is utterly meaningless, the two have barely interacted across two seasons, they don't have any connection to one-another.
The only logical reason for her story i can think of is that she's gonna be a central character in the upcoming Noxus show(the absolutely-not-subtle Swain raven snatching the hex crystal from Jayce's hammer strongly points to how this story is going to go). Her usefulness to
Arcane is neglibigle.