There should be no breaking changes from what i know?
What happens when you try to install
You need to atleast have 1 custom page using more dropdown to activate it before other modules page are able to use it
You must contact your host provider to get the database address if it not hosted on same machine, But you should try 127.0.0.1 or localhost
Sounds like the forum module is missing files, Have you modifyed/deleted anythings there?
Be sure you have setuped the permissions for each group and the url path starts with /