The correct answers are The Princess's missing tiara caused quite a scandal. and The commander-in-chief's horse refused to move.

Singular nouns ending in s (like Princess or Jones) usually take an apostrophe + s to show possession (Princess's, Mr. Jones's).

For compound nouns (like commander-in-chief or mother-in-law), the possessive apostrophe + s always goes at the very end of the phrase, not in the middle.