A sentence that starts with
It is hard to tell
and continues with a tensed embedded question object complement clause like
what would have occurred if [something ...]
is an example of what's called Extraposition in English. Extraposition is a construction that takes a sentence with a "heavy" subject complement clause, like
What would have occurred if [something ...] is hard to tell.
and "moves" the subject clause to the end of the sentence (where it's more comfortable, since English is right-branching), and leaves a dummy it behind to fool us into thinking that the sentence still has a subject.
It's hard to tell what would have occurred if [something ...].
Extraposition is one of many "movement" rules that place long complex constructions towards the end of the sentence, where they are more easily parsed and understood.
English Rule No.1 is
Every sentence must have a subject NP.
That's why there's a there in There's nothing here, and why there's an it in It's raining, and It's a long way to Tipperary, and It's hard to tell what else.