If you are willing to add a registry entry, you can set the following:
HKEY_CURRENT_USER\Software\Wolfram Research\Mathematica DisableKeypadEnter = DWORD:1 (DWORD is the type, not part of the value)
The enter key on the separate number pad of a keyboard works that way, the main enter key needs shift first.
I dont have number pad so...
What kind of keyboard are you using? What OS are you using? Does it have additional Numpad Enter key? Do you use "sticky keys" feature? Also take look at this link
I have a normal keyboard (my laptop is an HP Pavilion 11), my SO is Windows 10 and i dont have a Numpad, so i dont have an extra enter key. I've never use the sticky keys, but maybe it is acctivated so as soon as i can try out your hints i'll advice you