Hello again. Glad you got the query working. As for your other questions, I am not very familiar with the API your are using. I know more about the Velo APIs and I think you are using this API so my first suggestion is to look at the field definitions in the docs to see what they are telling you.
As far as abuse, this could be difficult and I assume will depend on what data you are collecting. I can make new email accounts to get free trials and you would have no way of knowing it was the same person again if the check is through email.
Sorry I do not know more, but perhaps someone in the community here has used the App APIs more.