I'll do my best to answer this question, but understand that it is a complicated one. If this stops being helpful, feel free to ignore it. There is a lot to learn if you are new to Dramatica. If this is your first time attempting to build a storyform for your story, the single most important thing you can do is be patient with yourself. This is going to take time; think of it like learning an instrument. It won't happen overnight.
Is TDE trying to "obtain a corrupted Luke," or is he trying to corrupt him? Getting Luke to draw increasingly on his Loki-self does not sound at ALL like obtaining to me. It is far too process-oriented (as opposed to state oriented) an endeavor.
When you say things like "Luke [is] gaining corruption," this sounds to me like you are twisting your story around signposts that don't fit. I guess you could characterize the process of a person being gradually corrupted as them "gaining corruption," but it's a stretch.
Moreover, I'm not sure the conflict is about who "obtains" Luke at this juncture of the story. It sounds like the conflict is more about who makes headway in convincing Luke to see things the way they do. And how do they do this? They're not trying to kidnap him, they're trying to win him over, presumably by conversation or argument or manipulation. Whatever obtaining might result or precede these endeavors would fall outside the bounds of this signpost.
And at the end of the day, none of these instances of "Obtaining" sound half as much like "Obtaining" as does "catching TDE and reclaiming the relic." Those are clearly state-oriented endeavors that result in capture or possession. In light of this description you provided of your story's ending, I would definitely say that Signpost 4 should be Obtaining, not signpost 2.