Or if you don't like silly internet stuff, here is the normal version of the song.
Kaj mi provas. Ho mia dio mi provas. Mi provas tuta tempo en ĉi tiu institucio.
I want to say yes. But it's probably something best reserved to friends. It would be short for "Saluton! Kio okazas?" which I think makes perfect sense.
"Kio okazas" is definitively NOT suitable as a greeting. Nobody uses it that way.
Funny. I just watched that same video before posting here. It's great, as long as people know it's a joke - not learning material. :-)
Why is it kio and not kion? I think it's kion because it should be in accusative
Why do you think it should be in the accusative?
It's the subject of the sentence.
Remember that you can usually ask after the subject with "who or what [verb]s?" -- and here you are literally asking "What is happening?", i.e. using "what?" to ask after the subject.
The subject of a sentence is in the nominative case, without an ending.