Most Images are encoded form 0 (black) to 1 (white).
The Z pass, being the distance from the camera to every given pixel, will contain values larger than 1, that's why it can't be enclosed in a regular image format. OpenEXR will contain the information perfectly, not only preserving the correct values but keeping it linear.
You are right in that the Z pass information cannot be visualized unless is normalized, ie. converting the lower and higher values to 0 and 1 respectively, and remapping everything in between. But the Z pass was not designed to be visualized...
So do yourself a favor and DO NOT MAP THE Z PASS INFORMATION using a Map Value node or Normalize node in order to save it as a "visual" representation, you'll end up distorting, compressing and ultimately loosing the accuracy of the Z-Depth information.
Instead, bring the Z pass intact into After Effects in a 32bit linear container (OpenEXR or OpenEXR multilayer). Make sure you enable the Z Buffer.
How do you set up your scene in After Effects to use the Z pass for 3D channels?
The use of the Z pass might vary depending on what you are intending to do with it.
If you must use it as visual information you could use the 3D EXtractoR and assign the color channels to use the Z information and set black and white points:
For more info on how to use Z depth... please ask in a site devoted
to After Effects...
Now, dealing with the second part of your question, on how to exprot the focus point. You can set up an empty to track focus in blender. Then you'd need to export the blender scene using this plugin: https://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Import-Export/Adobe_After_Effects. That will create a .jsx script that can be run in After Effects. The script will create a 3D composition with a camera and Null Objcts that correspond with the origin of the objects on the blender scene. You would then need to create some expression in After Effects to use null object that corresponds to the empty on the blender scene, and have the null control the focus point in the After Effects composition.... again, something to ask somewhere else...
More information on OpenEXR here: http://www.openexr.com/TechnicalIntroduction.pdf