Questions about example sentences with, and the definition and usage of "Ingress"

A: A place or means of access; an entrance

A: It means water had entered the site. Depending on the context, it could mean that the site flooded.

A: it’s a noun which means “going in”: very high register.
He refused to grant them ingress.

A: I downloaded "Ingress" instead of Pokemon GO because there are no Pokemon near my place. So I'm gonna be an Ingress agent! I'm registered.

I'm not positive what you meant by "registrant". It's definitely a word. If you wanted to use it, you'd have to have an article before it (a/an/the, but 'a' in this case)