Fords of Isen Battles Puzzles

I'm puzzled about two items about these battles from the account in Unfinished Tales:

1) In the First Battle: a battalion of Uruks drive Theodred's defeated riders south down the river. The Riders make a stand against them on a hill. Then a horn sounds and the enemy disappears. The last sentence says:

"It was not till later that they discovered whither the Uruks had gone."

Where had they gone?

2) In the Second Battle: Erkenbrand sets out from Helm's Deep after errand-riders from Elfhelm inform him of the defeat at the Fords.

I still cannot envisage where he went. How did he gather the scattered riders without encountering the huge invasion of Isengarders advancing on Helm's Deep from the Fords and from the east side of the river?