As much as what happened was wrong, and common sense did prevail, Hearts only have to look at Law 7.3 which specifically states "The additional time may be increased by the referee but not reduced" to know they potentially have reasonable grounds for appeal imho as clearly he did reduce the time if he blew up when he supposedly did, although I don't recall ever actually hearing the whistle, which suggests it was closer to being an abandonment than a completed game.
I think the most reasonable outcome is a big fine, games behind closed doors and maybe a points deduction of some form for Celtic, but in theory a replay is possible if the SFA/SPFL agree that the referee was erroneous in law to end the game.