this is not the real Nathan. Nathan would not have deferred to Jason, much less covered up the truth even if it was his gf's son. I'm honestly not sure I see the need for this cover up. Britt can testify that Cullum is dirty, has been blackmailing her and attacked her & Jason. Unless Cullum ditched the knife he used to stab Marco then it should still be on his person. Even the PCPD could make that case.
The writing feels pretty on the wall for Marco to me. All the flashbacks, the declarations of love, Lucas insisting "He is The One!". Clear soap signals for "He's a goner". But also the irony of Lucas saving Marco's killer while not knowing that Marco is dying nearby. Lucas will spiral, maybe pop some pills or hit the bottle. Oh, the drama.