(1) I think 晚 is used as a verb here, meaning "to be late". In Chinese an adjective is often used as a verb, for example, 你瘦了 (You have become thinner).
Remark. In this case we often say the adjective is used as a verb or as the predicate.
(2) 了 doesn't go tegother with 一. Here 晚了 is the past form, meaning "was late"
(3) If you add 来 to this sentence: 我晚来了一个半小时, then 来 becomes the verb and 晚 becomes an adverb to describe 来. But I wouldn't recommend to use 来 here because
She told me to get home by 11, but I came home late by an hour and a half
sounds a little weird with "came". So we better change it to "got", which corresponds to 到 in Chinese, that is, 我晚到了一个半小时
(4) 迟到 also means "to be late", but usually for something formal, like a meeting, a lecture or a movie, or for work. One wouldn't use 迟到 for going home, even if it is grammatically correct. However, 晚到 can be used for this situation.