I just had an idea for how they could take out Earendil. First they recover the Silmaril from the sea. Since Herumor is most likely Black Numenorean, he ought to be able to gather a fleet of Corsairs of Umbar (also descended from Black Numenoreans). With the power of the recovered Silmaril, they can sail to the edge of the world and wait for Earendil to pass over them. Again using the Silmaril, they call up all the waves of the seas and pile them up into an enormous mountain of water that lifts the fleet up high enough to attack Earendil.
It's possible that Herumor could end up with two Silmarils if he could pull that off. If he found the third, he might have enough power to split Arda wide open. The PCs better make sure that doesn't happen.
I'd say that the Silmaril of air ought to be retrieved and returned to the skies at the end of the adventure. Permanently losing the evening star is too big a change in the world, IMO. Whether or not Earendil and his ship Vingilot still bear the jewel afterwards is up to you. Personally, I can think of worse ways to retire a PC than to make him or her the new evening star.