[Lalaith]

Elempi, I'm sorry to hear you're going.

As for what the Borrim should do...historically, as it were, the sons of Bor (Borlad, Borlach and Borthand) followed Maedhros and Maglor, while the Ulflings marched with Caranthir.
It is important, canonically, that no elf that marches to the Nirnaeth should have any inkling as to the treachery of the Easterlings. So I suggest the following:
a. Fastarr and Embla, who know the truth, do not go back to the Borrim. I think they should be thoroughly terrified by the whole affair, and go perhaps to Embla's people.
b. The rest of the Borrim - Briga and co - would also bring troubled tidings, so perhaps they do not get back in time; the sons of Bor have already left with Maedhros and Maglor when they return?
c. Tathren - if he tells Caranthir of alleged Borrim treachery, why do the Borrim still march with Maedhros and co? Communication issues between the brethren?