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Considering that newer starters evolve into Pokemon that look very different, I would say that these designs are totally plausible.
The Chinese leaks, which have proven to be the most accurate and consistent, stated that the final evolutions are an archer, wrestler, and siren. These same leaks do not mention split evolutions or alternate forms.
Also, the girl in Litten's reference sheet was unable to be identified until the official announcement of Mallow. This, plus the reliability of the leaks, pretty much confirms these starters to be real.
Now, some might say that those reference sheets may have been scrapped, rejected, or revised. However, Mallow, Ash, and Pikachu all make appearances on those sheets. I wager these sheets are meant to help animators for the show, which means the show is already in production and these starters have been finalized.
I doubt such detailed reference sheets would be made for pokemon designs that were still awaiting approval. Usually a design is decided on first, then a reference sheet is made to help the animators. They decide on a design first, a design that all animators have to follow so that the character looks consistent throughout the show. Now they must implement the same design in the show. The character's expressions, posture, interaction with other characters, and other kinds of details are taken into account.
Personally, I like these designs very much. They're clever, unique, and really fun. A Robin Hood Rowlet, Tiger Mask Litten, and mermaid Popplio are all so refreshing and exciting.
