To enable right-click as Enter when some object is selected you need not only to check-mark "Enable quick secondary click as return key" in AutoCAD Preferences, but also fine-tune SHORTCUTMENU system variable.
When you enable quick secondary click as return key, the value of SHORTCUTMENU changes to 27, but you need to set it to 25, then right-click will not show context menu when object is selected, but rather will act as Return (Enter) key.
So you need:
1/ Enable quick secondary click as return key in AutoCAD Preferences as I described in my first post.
2/ Set system variable SHORTCUTMENU=25 - at the command prompt type SHORTCUTMENU, hit Return, then type 25 and hit Return again:
Command: SHORTCUTMENU
Enter new value for SHORTCUTMENU <27>: 25
Command: