Why is "er" needed here at all?
According to other threads, "er" is most often for phonetical convenience. What I don't understand is why it's unacceptable to omit it. Is "er" grammatically necessary in this sentence?
why can it not be "wat er gebeurt in de grot?" - with the "er" and "gebeurt" swapped around?
This is known as inversion, where the subject follows the verb. It is used for questions and for certain subclauses.
See here for more on inversion and Dutch word order.
Would you share the url, aub? Links don't seem to work on the app, unfortunately.
Your sentence translates as "what happens in the cave" as a statement, not as a question.