When Eowyn rode with the Rohirrim host to Minas Tirith as Dernhelm, was this the first time she had ever donned this disguise, or had she ridden with the Riders before?

"There seemed to be some understanding between Dernhelm and Elfhelm, the Marshal who commanded the éored in which they were riding."

How much did Elfhelm know and had Eowyn ever ridden with his men before? Who trained her to fight.

Did she ever use a sword again in the future?