I disagree that it means "faked". To me
To Wilkins’s presumed dismay and embarrassment
means
To what I presume to have been Wilkins's dismay and embarrassment.
or
To what we must presume to have been Wilkins's dismay and embarrassment.
In other words, whether or not Wilkins truly felt dismay and embarrassment, any observer of the situation must surely presume that he did.