I think Olympics/romance would be enough for the plotline. I also think you could do something with the fact that this will the first time they will on the same team since they were 12 I'm just not sure what (celebration dinner? renting a movie? something else?).

How long should it be from Rin's departure until he comes back?

2 years maybe. I don't really know shit about how this works, but I think that would be enough time for Rin to do everything that he wants to do in Australia.