Could they have been forgiven? Quite possibly. Maglor was sure sick of everything, and felt even if they were punished it would be just. It was only Maedhros's constant nagging that finally made Maglor go with it.
Though I could see all of Feanor's sons being in Mandos for a very long time. Maybe not as long as Feanor though, I don't know.
On a quick note, has anyone ever wondered why Eonwe simply let the brothers go, instead of just capturing them and taking the Silmaril's back? I mean in the grand scheme of things it makes sense, just sounds odd that he'd let them go.
[COLOR=#0000cd][B]Eodread, Earendel, Isilmacil - Horizon[/B][/COLOR]
[B][COLOR=#ff0000]Thattickles, Thangorodread - Table Smashers[/COLOR][/B]