I do not recall any earlier decks depicting her as a possible Demeter, but the iconic representation isn't such a jump.
The Empress, as Empress, is closely connected to the fertility of her realm, which is often symbolised with an ear of corn by a fall of water - here imaginatively recalling Demeter. The veil of the previous card may also connect the High Priestess to Persephone, and thus, her mother - the mother of the land/Empress - to Demeter.
From a different perspective, the image may, as images do, not be restricted to only this one possible connection. linked to Demeter through the fertility of the tilled land, but also to Aphrodite through her connection with the GD's allocation of Daleth and Venus.
The Dalet - 'doorway' - again making its connection to motherhood clear, for the mother is the doorway of incarnation/birth.
The move from the eagle upon the shield to the the Heart (or Venus) is, as mentioned above, because of the Qabalistic allocations made by the orders to which both A.E. Waite and Colman Smith were members.
On a different note, though I agree that Pamela Colman Smith has not been given due recognition for her gift, I do not think that it is correct to swing to the other extreme and even partially dismiss the precise instructions Waite would have given to the artist he, it must be remembered, employed to create a quite specific deck. How she worked with the instructions are of course a reflection of her creative talent, her understanding, and her attunement to the forces which worked through her, but also the guidance and directions of Waite.
As to what leaway she had, I suspect that I tend to agree with Kaplan with regards to the minor arcana - and that she was possibly given virtual free reign, though she was also, undoubtedly, influenced by the Sola Busca non-Tarot deck at the time on exhibition at the British Museum, by Qabalistic considerations from the Golden Dawn, and by comments in Papus's book.
With regards to the Majors, I do not think that Waite's instructions would have allowed much artistic licence.
But this probably better belongs to another thread - The Empress, to my mind at least, is still pretty close to its Marseilles precursor.