Q. With regard to this Q&A, I believe you have misanalyzed the meaning and etymology of the singular word corps.
Q. When referring to “the corps” as in the Army Corps of Engineers or the Peace Corps, what is the proper possessive form? For example, is it “the corps’ decision” or “the corps’s decision?”
A. Use corps’. Please see CMOS 7.19 for the possessive of nouns that are plural in form, singular in meaning.
You give a solution based on CMOS 7.19, but actually CMOS 7.16 applies to singular words ending in silent s (e.g., corps, Illinois, Jacques, rendezvous, chamois). CMOS 7.19 refers to words that are plural in form but singular in meaning. However, words like corps or chamois are not plural in form. The word corps, for example, is singular in both form and meaning. It comes from the French le corps. The s on the end is from the original Latin spelling corpus, which is also singular. The s does not signify plural and never has. (The Latin plural is corpora.) We use the invariable plural form that French does in spelling, but in English the singular is pronounced /kor/ and the plural, which happens to be spelled the same, is pronounced /korz/. Thus, by CMOS 7.16, the possessive forms should arguably be “the corps’s plan” (singular) and “the many corps’ plans” (plural).