I just checked the American Heritage Dic.and Collins, both have "in a shambles/ a shambles".Neither dictionary says anything about singular or plural. As both dictionaries have "a" one may assume that
"shambles" is a singular. But I take it that a lot of people think it is a plural. So I would see the situation with the American who said it should be "in shambles". It seems to be a dark idiom, even etymonline does not try to explain the s of shambes. Obviously one knows the origin of the word and the various steps in
the development of meanings, but there seems to be uncertainty about the s-ending.
In such cases it is helpful to find a hypothesis that might make the curiosity understandable. The semantic steps were
bench, bench of butchers, slaughterhouse (with butchers' benches), carnage, chaos/mess.
So, I think, one might assume a formula such as "a shamble's carnage/chaos" meaning the chaos of a slaughterhouse.
The genitive shamble's may easily have become shambles.
Please, don't take this for a historical explanation. It is only meant to give an idea of the curious things that can happen
in the development of languages.