Oliver is Felicity's plus one. It's a matter of public record that Bar and Fel are friends--and she helped out with his case when he was in a coma. And she goes to Central Perk--um--Jitters whenever she's in town--which is where Iris used to work and still hangs out.
The stand-off between Cliff and Bar was personal, from the first time they met. It was Allen not his secret identity who was ticked off with DeVoe. From a visual, visceral perspective it made more sense for Barry to beard the lion in his den than it did Flash. Not completely intelligent--Cliff put the suggestion in Barry's mind when they were at the police station--but it makes sense from a character perspective.