When Superman is fighting AMAZO, a crowd appears in the background between shots. The camera moves to a close up of Superman, where he exclaims: "that's not good" it cuts back to display both characters, and the crowd of dozens of people suddenly appears. Earlier in the same scene people are walking in the background casually as if nothing is happening.
Sherloque gives Superman a check for his ex-wife's alimony, which raises the question of if a check written on Earth 1 even be valid on Earth 38. But,, while the parallel universes phenomenon may not be public knowledge, Sherloque himself obviously is aware of them so, having been to several of them already, would simply be using checks issued from the very place they're intended for. He is after all supposed to be quite clever.
When Amazo analyzes and duplicates the quartet's powers and abilities, a tag reading "ACQUIRING META DATA" appears to him. Of the four, only the Flash is a metahuman; the others' abilities all come to them naturally.
Barry and Oliver travel to Earth-38 to seek Kara's help, but it's not clear how they know she's in Smallville. The more obvious place for them to look for her is National City; since Oliver has been in prison for seven months and Barry has been dealing with the arrivals of both Cicada and Nora, it's unlikely they've kept in touch with Kara enough to know she's been dismissed from the DEO, and since their friends at S.T.A.R. Labs don't yet believe they're telling the truth, it's improbable that Cisco would be willing to help and vibe her location. And since neither they nor Kara mention talking to her sister Alex, it can be inferred that they didn't check the most likely place to find her first.
To convince Iris about the truth of their situation, Barry suggests that Oliver talk to her and call her his lightning rod. Since Iris already believes that Oliver is Barry, this would only serve to reinforce that notion; she would more likely be convinced hearing it from "Oliver".