Such a fine edition and translation deserves (perhaps in a secondedition) a better packaging.
The majority of Scheil's critiques of desired material seems to have been filled in broadly by:
Henley, Georgia, and Joshua Byron Smith, eds. A Companion to Geoffrey of Monmouth. Brill’s Companions to European History 22. Brill, 2020. http://archive.org/details/oapen-20.500.12657-42537.
Obviously this isn't an inconsequential amount of scholarship (575+ pp) to have included in Reeve's volume.
While it's nice to identify what is not in the reviewed volume, it's probably better to frame it that way rather than to seemingly blame the authors/editors for not having included such a massive amount of work. This sort of poor framing is too often seen in the academic literature. Reporting on results and work and putting it out is much more valuable in the short and long term than worrying so much about what is not there. Authors should certainly self-identify open questions for their readers and create avenues to follow them up, but they don't need to be all things to all people.