27.) The phrase "a rejoint" refers to the past tense, through the passé compose. Since La Fayette joining his troops was something that already happened, it makes grammatical sense to use the passé compose. Furthermore, rejoindre is not a verb conjugated with être.
30. In this sentence, the control of the troops (commandment) is the direct object in the sentence. La Fayette is the Indirect object. Control is being given to him, therefore ''lui'' is used.