As far as lathe/mill WCS tables are concerned let's KISS.If there is going to be Abs/Inc entry, there needs to be a visual indication of the current mode.
Needless to say, this should *not* be tied to the similar entry mode selection over on the Offset Library screen, in the Lathe software.
Do not toggle between "absolute" and "incremental" input modes. Leave the "ENTER" key function as it does now were the typed value is written to the high-lighted entry.
Have a "SET" button that copies the current machine axis position to the high-lighted entry.
Have a "INCR ENTRY" button that adds the typed value to the high-lighted entry.
Have a "CLEAR WCS" button that clears the high-lighted WCS. For the nervous you might put in an "Are you sure?" prompt if you like.