Right, we know what happens in the book for these scenes. It's a question of...does Finarfin just announce his decision, say goodbye to his kids, and walk away? Or does he attempt to persuade them to return with him? Obviously Galadriel is still gung-ho "Let's go to Middle Earth!" - but did the Kinslaying make her reconsider the idea of going *anywhere* that Fëanor leads? Does Finrod want to go back with his father, but instead continues on to lead those of their people who are remaining? Does anybody discuss anything?
The Doom of Mandos does change something - it explicitly puts the possibility of betrayal on the table. This no doubt increases the uneasiness between Fëanor's and Fingolfin's camps. Does it come to the point of accusations? Paranoia? We also have not seen any of the Noldor's reactions to the Kinslaying yet (except Fingolfin, who broke his sword). The Doom is an opportunity to dredge up some of that guilt and see how they are handling it.
There are several 'tiers' to the commitment of the Noldor to continue on to Middle Earth despite the words of Mandos.
First, we have those who swore the Oath (Fëanor and his sons). If they abandon their quest now, they are consigning themselves to Everlasting Darkness (the Void). Thus, Mandos' words are less scary than the threat they've already called down on themselves. They are continuing no matter what.
Next, we have all those who are guilty of bloodshed during the Kinslaying (Fingolfin, Fingon, probably Celebrimbor, etc.). These elves know they will face judgement if they return. Not just at the hands of the Valar, but they will also have to face the (surviving) Teleri, and perhaps even the elves they killed (when they return from Mandos). In other words, they might be scared by Mandos' words, but they don't want to be in Valinor anymore, either.
Third, we have those Noldor who are guiltless in the Kinslaying (they weren't even there until the aftermath), and just really want to go to Middle Earth (because of all the previous reasons). The question for these is...is that still enough incentive to continue on despite Mandos' dire warning and after witnessing the extent of Fëanor's fanaticism? For many of Finarfin's host, the answer is clearly 'no,' and they turn back. But some continue (such as his children).
We know the result. What we don't yet know are the nuances of the decisions that are made. How do you choose between obeying the Valar and remaining loyal to family? How do you choose between holding onto a quest whose ideals you believe in, or abandoning a quest whose leader is showing some moral bankruptcy? As a leader, what is your responsibility towards your people - should you urge them to go on or urge them to turn back? What if they ignore you? That's the sort of stuff we have to work out.
If the Doom of Mandos changes *nothing*, and is merely an excuse for Finarfin to wimp out and abandon the quest...we're playing it wrong, or at the very least, missing an opportunity to do a *lot* more with that!