Hey Daniel,
Although I’m not an expert in Hemidactylus geckos, I can try to answer your questions. Regarding your first question, I unfortunately can’t give you a straight answer. Although they may be able to be differentiated by call, it’s much easier for identifiers to find the species from a photo. Often times, you need to pay extra attention to the scales when telling apart Hemidactylus. Maybe another expert can chime in for this question.
As for the second question, I took a look though the gecko observations in the DO, and most of them seem to be accurate, at least to genus. Like you mentioned, Hemidactylus geckos are pretty difficult to tell apart. There may be more species in the DO that haven’t yet been observed on iNat, but iNat at least shows you the four most commonly seen in the country.