1. off of 是英语习语，一种不太规范的用法，多见于美国英语。在本句中意思等于from.请参见https://www.lexico.com/definition/off_of

2.你的理解正确，句中的of可以用from替换，但第一个替换时需要同时去掉off. 

请看外媒相关例句：

1. Today you could pay $8 to $10 for a single movie ticket, and movie theaters make most of their money off of concessions, not the price of the ticket.

2. Cinemas keep not only a majority of the ticket money, but also all the profit from concessions. 

3. In 1964, 6 percent of organizing committee revenues came from worldwide television rights and 18.9 percent from ticket sales.