Is a branch retinal vein occlusion could be a sign of MS?

Don't think so. Optic neuritis is typically associated with ms. Brvo is associated with hypertension, or certain blood clotting disorders, depending on your age.
Not at all. You are describing a cause of an ischemic optic neuritis, which is very different from an inflammatory optic neuritis. MS is not a disease of blood vessels or circulation.
No. Brvo is not typically associated with ms. You need to be evaluated for systemic hypertension, diabetes, clotting disorders, and other related eye/ systemic disorders to prevent recurrence and involvement of your other eye. You also need to see a retinal specialist to treat the brvo and minimize long term related deficits.