Outlining with the keyboard works fine: press tab on a task and it becomes a sub-task. This does not work with the mouse: you can only drag tasks into projects, not other tasks.
This is by design at the moment.
But when I get a chance (post 3.0 release) and add the option to hold down shift when dragging to drop ontop of an item like you could in TaskPaper 2.
Why not use two trop targets? That already works for items inside projects, though the second one is a little weird, as the text ends up nowhere near the red line but as a new aunt.
I feel like two drop targets starts to feel to finicky. But in the latest 3.1 dev release you can hold shift down to drop ON.