Yes, I get what you mean now. I do think I got wires crossed as to Morgoth/Sauron motivations in what you said. This I agree with as Sauron motivation.
The things this approach I described above would accomplish are:
1) to stress the impact of Fingollfin's deed
2) to give our villains a believable conflict of interest
3) to explain why Dorthonion is vital for Morgoth at the moment and
4) why the orc road build though Dorthonion leading to Doriath is called "Orcs' Road of Haste"
5) the building of which road gives the orcs the reason to have been there in Dorthonion to be ambushed by Barahir's men in great numbers o warrant their reputation as orc-killers
6) to explain why Morgoth placed a price on Beren's head without thinking too much of him personally
7) to explain why Sauron is so little interested in Beren himself when they do meet finally
8) to have Thuringwethil be the poor messenger who Sauron sends to face a more and more annoyed Morgoth as she has to stand in and explain Sauron's apparent negligence or incompetence in clearing and claiming the whole of Dorthonion and moving along with the Doriath attack plan
9) to set up Thuringwethil's fury at finding out that while she had to be covering in front of Morgoth for Sauron's out of a sudden losing interest in "doing his homework", he was not only "playing with his new play station instead" but he was "playing with HER very own play station he appropriated for himself behind her back without her even knowing"
10) to explain why Sauron does claim Dorthonion after the explosion of the "tub of evil" before he moves on to future plans behind the mountains - the Doriath attack part is shelved for now due to Morgoth's need to recover and to deal with Maedhros encouraged after the B&L success, but Sauron finishes the Dorthonion part that has been his duty.
We do not have to show all of such a dynamic, but what we show should be making sense. We should take our villains seriously.
Such a dynamic would also explain Thingol's being on the edge, overstrained and impatient with both wife and daughter. He has more pressing things to deal with now than his wife's cryptic warnings or his daughter's ridiculous new choice of a lover ("This one at least will die before she gets bored with him, .like she did with the previous one, but still, she will be heartbroken. Let's sends him away and they will both come to their senses when apart for a while. I really have no leisure to reason with her now.)