I would have a site with following specs:

- Mainsite ONLY with Userdetails (shared to all other sites) - MASTER
- Community (with Eventmanagement) - SLAVE
- Blog / Some User are Blogger - Events link (Modul from Community) - SLAVE
- Forum - Events link (Modul from Community) -

each site have a different domainname.

Yes you can do such kind of things.

Remember that the concept of JMS is based on the sharing.
- Sharing of the same Joomla application (php code)
- Sharing of the content of a limited number of extension (MySQL tables).

So you will need to have all the extensions installed on the master to have the PHP code.
When you share the content (ie. Joomla user) then you have exactly the same info in all the website and any change in each slave of a user will automatically available on the other site as the data is unique and shared.

You can effectivelly create wehsite into different domain, subdomains or subdirectories.