I think this method would be unfair for both the networks and the viewer. The bulletins all air on this service exactly half an hour after they air on their respective home stations. No one network has the advantage of starting earlier than their competitor(s). In your hypothetical, WIN News is pushed back to starting at 8.30pm, 9pm in some cases, 9.30pm if Prime7 News goes hybrid, and so on.
There will, however, always be a problem with bulletins starting too late with this service. I think the best way to minimise this is to give the least popular/smaller market ones a late start
Imparja has added and removed the bulletin once before, so maybe keeping it would be a good idea.
I like the idea of adding channels for metro bulletins. They could be sorted quite uniquely due to the timezone differences. For example:
421: Nine News Sydney live, Nine News Perth live
422: Nine News Melbourne live, Nine News Darwin live during DST only
423: Nine News Brisbane live, Nine News Adelaide live during DST only