Hence we go back to why my definition of corruption (as an active conduct) cares about method and intent.Okay, I hear where you're coming from. Obviously the actual criminals are guilty while others can share responsibility but not the blame, it's not even a discussion. I guess our difference lies in who we see as the criminal in this analogy. You think it's the person doing the corruption, but I see the person being corrupted as a future criminal on their villain arc (IF they end up doing bad / irresponsible things according to their own starting morality). And because the person is on their villain arc I just don't see much point in finding the person guilty for making them a criminal (aka "We live in a society"), criminals aren't always pressured or coerced to do the crime, they choose to act based on external influence and their internal justification but they can only blame themselves for making that choice to go through with the crime.
One last thing (it's too engaging). If you think of a person doing the corrupting as a criminal it paints the person getting corrupted as an innocent victim which robs them of agency. They're a human being with their own agency and they're making a choice when they do bad or irresponsible things. Lena can't blame Seymour for making her cheat on Ian (the moral action here is either to break up with Ian or just be honest with him and explain her problem if he promises to keep it a secret) or blame Jeremy's dick for making her treat Louise like trash and gaslight her into becoming a cuckquean. Just like Jeremy can't blame Lena for making him a bad friend to Ian or poor boyfriend to Louise if he can't man up to reject her and do the right thing. Just like Ian can't blame Cindy for being sexy or Wade for being a bad boyfriend or Axel for making his moves on her if he ends up banging Cindy and betraying Wade. Well, they can but it just wouldn't be mature or convincing.
Method because manipulation, coercion, and any other forms of fraud or duress already rob a person of their agency, maybe not completely so and you can still imagine all situations to have a "correct moral solution" to them (specially with the power of hindsight), but at some point you will be just "demanding perfection" and blaming the victim, as well as adopting the callous doctrine that every failure is a choice. It can be hard to draw an exact line of where there is in fact mutual blame from the passive part of an interaction.
Intent because no one should be blamed for the influence they exert just by being. That would be an undue obstacle to their own right to self-realization (which is the whole problem with corruption in the first place). What I mean by this is, with the way Ivy dresses, conducts herself, and expresses her opinions, she is still capable of making an impression and it should be possible for that impression alone to influence another person into emulating her lifestyle, even if it meant a radical change to their own. I wouldn't call that "corruption", in the same way I don't consider Lena's interactions with Holly to be corruptive. The problem is that Ivy doesn't change Holly just by being an example, she doesn't really respect Holly as a peer and demeans her lifestyle, while embellishing and being completely acritical of her own. She's pushy and wants to change Holly regardless of what Holly's own thoughts and feelings are because she already dismissed them as being "naive".
I like that you brought the BBC storyline up because, obviously only up until the cuckqueen part of it, Jeremy really wasn't responsible for any kind of corruption even while carrying out behaviors that many would consider immoral. Jeremy did nothing to Lena, she spied on Jeremy and Louise and developed her obsessive fetish all on her own. He was lying to and manipulating Louise to try to sleep around, but until the cuckqueen part he wasn't really trying to change her in any way (it is possible that he pressured her into agreeing to things like filming her, but I don't think that's ever established so just as likely that Louise agreed to it because she was not opposed). After the cuckqueen plot though Jeremy is definitely Lena's partner in crime when it comes to corrupting Louise, they both know how attached and emotionally fragile she is and they take advantage of it to get her to "agree" to things that go against her established values.
Now about Lena corrupting Jeremy I would say that's a bit more of a grey area, I would agree that merely hitting on someone or trying to seduce them does not count as any kind of manipulation as long as you stop at the "no", or at any other expression that the engagement in sexual developments is uncomfortable to the person being seduced. On the other hand Ivy tells Lena how easy it is to manipulate Jeremy and orchestrates the events of that evening with the express purpose of getting Lena and Jeremy intimate and Lena chooses to take full advantage of that. I would say that in both cases Jeremy is still mostly responsible for his moral failings because very little was done to rob him of his agency, but Lena was only "corrupting" him in the scenario where he at least tries to say no (high Ian_Jeremy relationship and Ian has feelings for Lena) because Lena pushes through his attempts to hold to his values of not cucking his bro, even if they were rather feeble attempts.