Disguise

Emery's options are to either impersonate Arthur and let the wedding ceremony proceed as usual, or let the ceremony get canceled together with the news that the Divine Order has been wiped out, which would cause mass hysteria on top of everything else brewing in the background.

The enemies were already knocking at their gates, Arthur thought the mass hysteria would be nothing short of catastrophic. Hence Arthur asked him to pretend.

So much for the knights' oath of honesty and truth, Emery thought to himself. 

For a moment, he did think of Gwen and how some people would try to harm her at the wedding. The last word of Maleagant made him worry. If Meave was involved in this, anything could happen, he needed to be wary. Hence he agreed with the idea of impersonating Arthur. This way, he would be able to stand guard next to her and have access to order all the Logress knights around. It would be the best solution to the current situation.