When PowerShell 3.0 is installed, I can force PowerShell to start using the version 2.0
-Version
Starts the specified version of Windows PowerShell.
Ente
Tweaking the config file won't help, powershell_ise.exe definitely depends on .Net V4. It also depends heavily on the V3 version of the PowerShell engine.
There is no supported way to run V2 of the PowerShell ISE after PowerShell V3 is installed. Unlike the core PowerShell binaries (like System.Management.Automation.dll) I think the V2 ISE binaries are either overwritten or removed as part of the V3 installation.
I'm afraid you'll have to run your script from powershell.exe.