1. yes
2. depends on the tone you are using with the person
3/4. Depends on the nature of the joke.
it all comes down to "just don't be an ass". Saying "you all fucking suck" is bad. "kill yourself faggot" is bad. "wow learn to play u shitty hunter" is bad. "Look into some guides, you're playing bad" is fine. Racist jokes might be debatable and a grey area, but you can simply not do that. "Anal [Shadow Infusion]" is IMO fine, but might be debatable; it would probably be fine. Unlike "Why don't you [Feign Death] but do it for real" which would be bad(inciting suicide). "Buy character boost for 100USD" would be really bad too.
There's very very little grey area for "abuse". Don't be an ass and you'll be fine, really.