Infidelity in GG -- seems rather more like a motif (a recurring idea) when you see that not only do Tom and Myrtle cheat on their spouses, but that Daisy also cheats on her spouse. Furthermore, many of the partygoers are depicted as switching from one person to the other and rarely ending up with the person with whom they came to the party. This is only a depressing comment on social relationships unless you can also pair it with Fitzgerald's exploration of post-WWI attitudes towards sexual freedom and decadence. It is important to note that technically Gatsby is not 'cheating' with Daisy; he sees himself as 'wedded' to her from the night of their first kiss in Louisville and thus, by that definition, he sees the intervening years of her relationship with Tom as the actual affair from his 'own' marriage to her (for which he readily 'forgives' her). It is this supremely naive but idealistic romanticism that both appeals to Nick while simultaneously placing him in awe of Gatsby's misplaced passion.
2. Time - as a motif or as a structural device? you need to be more specific in your questioning techniques. Nick uses time to frame the narrative, in Chapter 1, when he says that the events which he is going to relate happened two years previously -- he already knows the 'ending' so to speak. And since he speaks of Gatsby in the past tense, so do we. The somewhat non-chronological format of the plot is a common modernist technique that Fitzgeral was beginning to explore at the time. It certainly can be seen as a weakness in the novel itself but some people enjoy the 'hints' that the text gives the reader as to the outcome this way.