Ok, I see what you're trying to do. You have a few options.
1) You could create a mapping to your includes directory in the ColdFusion administrator. The administrator is where you change all server settings, just like what the php.ini file does, except it's a nice web based interface. The only thing is, you'll need to know the administrator password. To access the administrator, it's http://www.yourhosthere.com/CFIDE/administrator (Just obviously replace www.yourhosthere.com with your host).
Under "Server Settings" -> "Mappings", you'll be able to add a logical path, such as '/includes', and map it directly to a system directory, such as 'C:\inetpub\wwwroot\yoursite\someSubDir\includes' (or wherever your directory is).
Then, you'll be able to access this directory just like if it were in the web root, with
ColdFusion will find the mapping, and find your file.
2) You can use the variable method, like you wanted to do originally. This is obviously as simple as just creating a variable for the path to your includes file.
If you want to only set this variable once, and in one place (so it's easy to maintain), you can put it in an Application.cfm file. Application.cfm is a special file that gets automatically included at the beginning of every request to a cfm page. If you're working on a pre-existing app, there is probably one either in the directory that you are working in, or in a parent directory. ColdFusion looks up the directory tree until it finds one to include for a request.
<!--- Set a variable to point to the includes directory --->
<cfset includesDirectory = "/subDir/myCFsite/includes">
<!--- Include a file from the includes directory --->
So you could set the variable in that Application.cfm file, and it will be available to all pages where the Application.cfm file lives, and its sub directories. Then, when you want to change the includes path, you'd only have to change it in one place.
One thing though, your app might use the newer Application.cfc file instead of an Application.cfm file. That is the more object oriented approach (CFC's are ColdFusion's classes). If so, I'll walk you through setting a request variable in that, but most apps have Application.cfm files.
Ok, with your changing of the title tag in your header file, you're basically going to have to use a variable in the header code, and set that variable before you include the file.
<cfset pageTitle = "My Title">
In the example, I provided a way to give a default title if the pageTitle variable is undefined when the header template is included. The cfoutput tags are just for outputting ColdFusion variables onto a page.
<cfif NOT isDefined( 'pageTitle' )>
<cfset pageTitle = "Default Title">