Would 'Er det nok?' be OK?
Yes, and it is and was accepted.
Are those sentences always interchangeble?
Is 'Er det holder' also correct?
No, I don't think so. "Holder" is the verb here.
"Er det nok?" would be a possibility I think. In that case "er" is the verb.
Ah, I'd forgotten about the V2 rule. Thank you. :)
Is there a difference between "holder" and "nok"? Thanks for the answer!
I think that Holder is a verb and can't be used as Nok .