Keira Knightley as Guinevere ready for battle. I think that the Picts could have gone to battle in very light armour such as these, but in the case of the women, the breast protectors would've wanted to protect all their breasts, not to show them off. That's not the aim of a breast protector! Apart from that, it's realistic for a very light warrior.
Keira Knightley would be a convincing Desdemona because she is able to portray so much emotion when it comes to tragic deaths, and given her acting repetoir, I think she would be the most experienced to play Desdemona.