As written, the story of Beren and Lúthien reveals the point of view of the protagonists, but doesn't spend much time developing any nuance to the reactions of other characters - Lúthien is a foil, and therefore characters respond to her beauty with various levels of selfless love or possessive lust. Which is fine for a stand alone story, but we will need our characters to be consistent with who they are outside of the Beren and Lúthien story. Thingol's disapproval can't come out of nowhere, but needs to be based on the story we have been telling to this point, and likewise needs to fit with the trajectory of the story we intend to keep telling after this season.
Celegorm has been a fairly likeable guy until now. He's mildly arrogant, but he tends to stay out of the political unrest and rather spends time with his dog. He can be steered by Curufin, but he hasn't been casually malicious. So, we can have him exhibit new behavior in response to meeting Lúthien, but it shouldn't be radically different from who he has been to this point. He and Curufin actively scheming to kill Finrod off and lock Lúthien up, forcing her into marriage... it's a bit far.
So, we went with the 'that's how Lúthien sees it' more than 'that's what they intended'. We have them reasonable (though still very self-serving), so that the audience can follow their actions without excusing them. And Lúthien has every right to feel betrayed, because she went with them to Nargothrond with the promise of aid, and then when she got there, they decided not to go.
