I have not found a datasheet for the MP1584EN switching power supply board. I have found information that there are multiple manufacturers, so information is really needed for the specific case to be accurate. However, the datasheet for the MP1584EN chip (not the board) shows a single vin, single vout, single ground. Which leads me to believe that that the 8 terminals on the board are actually only 3 different electrical connections. I expect that the 2 in- and 2 out- are all hooked together as gnd, and the 2 in+ are hooked together, and the 2 out+ are hooked together. Which matches the description of what you found.
Assuming that is true, the part file should reflect that by using 3 bus definitions. The extra terminals on the board are useful in cases where multiple wires needed to soldered on, but if you are putting it on a breadboard, only one of each bus is really needed. Or the breadboard can explicitly connect the common pins together.
For the project, the effect of all that is that there is no needed to route traces on the pcb around each other for the out- connectors. They are (SHOULD be) effectively the same pin. Connect the traces to which ever is most convenient, including only using one of the pins, and/or adding a trace between them.
Vias will not need any soldering. There are connected through to both sides of the board when the pcb is manufactured.
The total project can not exceed the power that can be supplied by the board. Connecting the board 5V to Nano VIN would cause problems because the nano would not actually get the 5V it needs internally. That internal regulator needs at least an extra half volt input to regulate properly. Specs say at least an extra volt. Leaving Nano VIN open, and connecting Board 5V to Nano 5V is not a problem. The “parallel” is not a problem.
On the schematic view, the normal convention puts the power symbols at the top, and ground at the bottom. You put them both at the bottom, with is easiest here. However, you can make that look at bit better. The power label parts can be flipped and rotated like other parts. If you flip the 3.3V (v++) parts at the bottom, they will connect better to the rj45 jacks, and the label ends up below instead of across the connecting wire. The same can be done for the 5V power on the KY41 header, allowing the schematic layout to be simplified. Labels should be moved, so that they do not conflict with the wire traces. Whereever that is practical. With the expected pin connections (and your test results) the power supply part of the schematic (and pcb) can be simplified.
On schematic view, traces are routed for clarity. On the pcb, when practical, they should be run more directly, with fewer corners. I doubt that the signal frequencies being used for this will cause problems, but with higher frequencies sharp (90°) bends are to be avoided. They can cause signal loss and RFI (radio frequency interference).
The pcb can be shrunk some more. I believe your main constraint/goal for the board, is to have the 4 rj45 jacks lined up for easy access. The rest is very flexible. In my previous version, I was deliberately not moving any of the parts (changing the layout), was not overlapping top and bottom traces to keep the routing more visible, and mostly keeping bottom traces horizontal and top vertical. Removing those constraints leads to a more compact board. I have stayed with a 0.05 grid size for this. The trace spacing (hand therefore parts) could be tighten more by reducing that further, and still stay within the constraints needed by design rules checks, and the needs of the pcb manufacturing houses, allowing the board to be a little smaller still. Which can be important when they charge by the area of the board. Even moving some of the silkscreen labels can help.
Here are a couple more variations to use for examples. To better view the pcb traces, toggle the display between “Both Layers”, “Bottom Layer”, and “Top Layer”. Using “Bottom Layer” seems to best to view everything at once. The top layer is dimmed (partly transparent), but routing is still visible. If you switch to “View from below”, that reverses, with the “Top Layer” showing everything.
heimdallRunes-3.fzz (56.1 KB)
heimdallRunes-4.fzz (58.6 KB)