Why isn't the "pa" sentence accepted?
I believe it's because they want the translation of 'father' specifically, instead of 'dad'. o uo
What's the difference between "opa" and "grootvader"?
Opa is more commonly used and more informal. I don't know anyone who would still use 'grootvader', not even my father. It is more common though if you talk about your 'overgrootvader', your great-grandfather.
There is none. Grootvader is more polite/oldfashioned, might be more common in Flanders as well.
Could you use a construction like "Mijn grootvader is mijn vader zijn vader"? I think I've heard something similar once, like "Saskia's book = Saskia haar boek", but I'm not sure.
At least colloquial that's totally accepted
mijn opa is mijn vader's vader.. nothing wrong with that..
Perhaps(?) because vader and grootvader are for speaking about relationship, and pa and opa are for referring directly to the persons.