Would Battenberg prepare plans for unlikely wars?

Would Battenberg prepare battle plans and weapons for unlikely contingencies such as a war with the United States?

Yes. Professional staffs maintain plans for dangerous worst cases regardless of how unlikely or politically undesirable they are.

A plan on the shelf does not mean the government expects or wants to use it. Battenberg's administrative system would still examine worst cases, because an improbable crisis can demand immediate action if it occurs. A hypothetical communist takeover in Ireland during the Cold War illustrates the same planning problem, while an imagined hostile British government supplies the reciprocal Irish example. These are conjectures about the plans governments would maintain, and the existence of the particular British plan had not been checked.