I have set up notification for presence sensors, but really don't like the default choice of using "departed" when people leave. Is there a way to edit the default text string to something less morbid?
With most apps, yes. How depends on what particular app you are using. What does your app setup look like?
I didn't even think of splitting them into different notifications, DUH! I just have them both in a single notification rule and have been sending the default %text% that is sent with each event to specify arrival and leaving.