I know a fair bit about biology, so I feel that I can suggest answers adequately.
I would describe their gemheart as a useless bit of "left over" material in their body that is created as a result of accrued minerals. Alternatively, one could say that the gemheart is a sort of useless "bone", such as the tailbone, found in humans, that serves little purpose. However, offering a satisfying explanation for the existence of the gemheart (why it evolved in the Parshendi and how it channels Investiture) seems challenging.
As the poster before me said, I would instead delve into how they use a specialized form of communication, and how they can pupate into new forms if the community needs them to do so.
Furthermore, one can surmise (from evidence on Roshar) that organisms (including Parshendi) evolved gemhearts in order to facilitate the storing and tapping of investiture. This cannot be explained satisfactorily through the use of non-Cosmere, real-world science without utilizing speculation and adaptation that replaces Investiture with electromagnetic force or something else. And even that is stretching the science a bit.
I apologize, but I cannot offer insight on how Parshendi are born.
Best of luck with your project!