In relation to the NRL Grand Final, any free to air network who winds up broadcasting it now or in the future is going to want a night time game. Reason is that, for FTA networks, Sunday nights is prime time for ratings and consequently for advertising rates charged. This that is where they make their money from, advertising. It also gives them a night where they know what its programming will be and thus able to save high rating shows to fill other dates.
I'm not condoning it pert se, but I can't see any FTA network changing from that view.
In relation to the NRLW Grand Final, I can't see it being a stand alone event in the short to medium term. Mainly this is due to the the crowd sizes that have been generated at stand alone NRLW fixtures. I'm sure a grand final would see a bigger crowd for the NRLW, but big enough to reach "break even" financially for the NRL, or even to make a profit? That's where I doubt it. As such, to save costs, the NRL will have the NRLW Grand Final on the same day and same venue as the NRL Grand Final.