1) Why do you sometimes categorize people as gods and other times as deities? Don’t they mean the same thing?

Indeed they do! You’ll see deity used in more scholarly texts and often in conjecture with cultures that had multiple beings they worshipped for similar things. There wasn’t just one Celtic river god, it was more like fifty. On this site, we’re using the terms god and deity to help us classify beings that were worshipped. Gods are the big names, those who most of the culture worshiped and had a huge amount of influence on the society. Deities are the small names, usually worshipped by a small group of locals or accessories to larger stories.  For example, in Greek myths we’d classify Poseidon as a god, but the individual muses as deities.