How can I make the enter key on my extended keyboard act as return only?
You've got your assumption a tiny bit mixed up. It's not that some software remaps the enter into return, it's that some software treats an enter as a carriage return and some doesn't.

They are two different keys with two differnet histories. One is enter and one is carriage return and they perform different functions.

I hate referencing wikipedia but hell, here ya go ... Differences between Enter and Return.

Usually most programs assume they are the same key - some don't. The AppleScript Editor for example uses &#x2318;+enter to force a script parse.