Dress will be found somewhere stashed in the castle.
Gwyn's alibi was that she was asleep and never left her room - a Camlannian maid will corroborate this maybe Wednesday
Gwyn catches a soldier and a maid getting it on from her window?
A strand or two of long, dark hair might be found on Samael's clothes and on the dress. These will match as they're both Leuko's. Strands of hair may be taken from a brush of Gwyn's to compare. Compared to Leuko's, Gwyn's will have different length, color, and waviness.