Answer: P. falciparum gametocytes with altered morphology due to prolonged storage in EDTA.
Careful review of the original slides revealed only P. falciparum parasites, and PCR testing for the 5 most commonly detected Plasmodium species revealed only P. falciparum DNA.
Also note that there are some hints that the rounded gametocytes are actually the same as the crescent/elliptical ones. Look at the image below which shows both forms. They are very similiar in appearance, despite the differences in overall shape. Therefore, you can see that a spectrum of morphologic changes occur overtime, with some gametocytes appearing more elliptical rather than round/oval.
This is an important pitfall that parasitologists need to be familiar with, and a good reminder for why blood smears need to be made as soon as possible after obtaining blood. Not only do the crescent-shaped gametocytes of P. falciparum tend to round up and look like gametocytes of P. falciparum with prolonged exposure to EDTA, but other changes can occur such as formation of 'applique' forms with P. vivax, maturation of gametocytes, exflagellation of microgametes, and rarely, formation of ookinetes (from fertilization of a macrogamete by a microgamete).
Thank you for all of the excellent comments!