P: Exit Keyword Entry by pressing the Return Key when the field is empty.

I was curious, so I tested Mac LR 8.4.1 and LR 9.1, and both behave the same as 12.1 -- double-return doesn't move the focus away from the keyword box.  In some ways, that's too bad -- it it were a relatively recent change of behavior, you could file a bug report about it, which sometimes causes Adobe to pay a little more attention.

Correct on all fronts - I am running LRC 12.1 on MacOS and I am referring the Keyword Entry field that gets activated with CMD-K.  I recall typing a Return when this field was empty to deactivate it but certainly not the case in 12.1 or the prior release.  The larger Keyword Entry field activated with Shift-CMD-K does behave as desired - Return deativates it.  I recall the single line field worked similarly - first Return accepted the input and cleared the field and a Return on the empty field deactivated the field. 

I am constantly chasing unintended keywords in my library now like "D", "E","/", "R", etc. 

To test my understanding: You type Ctrl / Cmd K, the keyboard focus moves to the "Click here to add  keywords" box of the Keywording panel, you enter a keyword, type return, possibly repeat, and then you'd like to type a final return to move the focus away from the box.  This would ensure that if you type other keyboard shortcuts, e.g. D to go to Develop, they don't get entered into the box as a new keyboard.

Are you on Windows or Mac?

On my Windows LR 12.1, the second return does move focus out of the box.  But on my Mac LR 12.1, the second and subsequent returns are ignored and focus stays in the box.

(Note that if LR displays one or more auto-completions for what you've typed, you'll have to type an additional return to select the completion. So on Windows, it could take three returns to move focus from the box.)