How can I distribute the flow from a catchment area evenly over all my inlets/manholes in the surrounding area, or evenly across a length of channel / swale?
To accomplish this, you would need to split the catchment into additional individual smaller catchment areas, each with its respective outflow node set to the closest discharge point. For a channel / swale using the cross section and channel elements, you could add additional cross section nodes along the channel if needed.
Catchments have a single "outflow node", as a catchment represents the area which discharges to a common point. If you have a situation where runoff is distributed evenly over a length of channel/swale, consider including the swale/channel itself as part of the overall "catchment area", with the "outflow node" as the common downstream point that the channel discharges to.
Another option to consider is a vegetative swale low impact development (LID) control. See: