On a search for journals, how can you return mainline only? I have mainline true, but it is returning every line

Every line on a JE is a main line, there is no sub-list unlike other record types

Is there a way to return a single line for a whole journal entry? Ideally I'd like to look at it in a sublist view?

Assuming you have very standardized rules for what is posted to each line of a JE, you could run a Line = 0 criteria to only take the first line