How do I disable mouse magnet on middle edge with multi monitors?

There is a setting for it in Displays:

You may still get the behaviour if you have a launcher on the right monitor, so use the "Launcher Placement" option to select having your launcher on only the left screen. The stickyness between the screens is required for it to be easy to retrieve the launcher there.

You might need to log out and back in (or restart) for the changes to take effect.

The "hang" is a feature to make it possible to use the launcher in multi-monitor setups when it's set to auto-hide. It can be configured using gconf-editor. The relevant keys are

/apps/compiz-1/plugins/unityshell/screen0/options/stop_velocity, and

/apps/compiz-1/plugins/unityshell/screen0/options/overcome_pressure

Recommended settings to minimize the effect are:

• overcome_pressure=1
• stop_velocity=20

These settings do not completely stop the mouse from sticking, but makes it less likely it will do so. You can reduce stop_velocity further if you want.

Possibly also /apps/compiz-1/plugins/unityshell/screen0/options/reveal_pressure. The latter can be configured more easily in System Settings -> Appearance -> Behavior.

• How do I use the gconf editor?

You can also modify the settings inside of CCSM if you prefer a GUI option. Its under CSSM > Unity > Experimental

There are options for Launcher Edge Stop Overcome Pressure and Edge stop Velocity which you can modify.