Australian Court confirms Instagram, Facebook waived right to arbitration by actively utilising litigation machinery during year long court defence

Can a party who seeks a stay in favour of arbitration almost one year into defending court proceedings be said to have waived its right to arbitrate?

Does US law require the party opposing the stay to clearly and convincingly prove that it would suffer prejudice if the dispute was referred to arbitration?

Are these questions best determined by an arbitrator (in line with the Kompetenz–Kompetenz principle) or by the court before which the stay is sought?