I want to make a wordpress site for an organization (main) that has many departments. The requirements are

1. There is only one website ( or maybe same theme for the main site and its department site)
2. There are some categories such as news, events or other articles.
3. If writer from department post a news, it has to appear on main site in news section.
4. If writer from department post an event, it has to appear as well on main site but in event section.
5. User can visit department site that only contains news and events about that department
6. A Writer can post a news and event for main site that is not related to any departments.

I have considered to use Multisite but the challenge is I can't find any plugins to retrieve post from another site for specific category (news or event). 

If I use a single wordpress, I have to think about how to display single page (I assume department is category or tag) with specific news and event widget that related to the department. 

**What is the best way in wordpress to achieve those requirements?**