First, read blogs about blogging. People who are earning a living from blogging. A good place to start is Blog Tips to Help You Make Money Blogging - ProBlogger
Second. Space your questions when you write a post, there is nothing harder to read and respond to than a block of text.
The answer to all of your other questions is:
Different audiences click more often.
Different audiences read more posts per week. (I think consistency is important)
Different topics have bigger audiences.
There is a difference between running a blog for money and running a blog to provide value, but you also make money from.
As for the forum, I haven't set one up so I wouldn't know. But it would make sense to:
a) Have a lot of traffic (users) on your blog
b) Encourage (give a reason) people to use the forum. (atleast let them know it exists)
Good luck defining your purpose.