What are some differences between the ways/contexts in which one would use "ephemeral" versus "transient", and vice versa?
Both refer to short-lived things or activities or events. Transient implies some kind of transition between states in which the transient state does not last very long.
I'd say that while "ephemeral" definitely means that lasts very little, "transient" can also take the hue of something transitory. And, sometimes, "transitory" things do become permanent :-)
Something ephemeral is, instead, going to die (soon) in any case. Sometimes lasting only one day, like the mayfly.