in the Qal is probably irrelevant
to the conversation as it is an unrelated (though similarly spelled) verb as opposed to the noun which is under discussion.
The inclusion in the LXX and 4QSam-b of a reference to "eating" may simply be an insertion into the text to make clear what is only otherwise implied. After all, what other reason could there be for David to want bread, if not to eat it. English translations of the Scriptures often include words not found in the text in order to make the text clear. It shouldn't surprise us if the translators of the LXX did the same. Nor should it surprise us if later copyists of the Hebrew also did this.