For 3. genenally older motherboards have less requirements for PSU quality.
But yes, problem may be motherboard or even CPU. I hope that RMA gets something sorted out.
It's supposed to work like "plug and play" -style so that's why it's somewhat less advanced than COM-ports in certain ways. Adding led that tells something is connected like on RJ-45 LAN port would be good idea but it adds cost (very little but still) so not many like to add it.
I suggest better PSU. Also while tose items should work together, I don't recommend them as there are better options available. Something like Ryzen 5 1400 + MSI B350M GAMING PRO combo.
Many parts are missing and I also recommend AM4 platform as it allows much better upgrade path. Ryzen 3 is coming in about week and is very affordable part.
Display takes graphic processing from GT520. There might be some problems with dual video cards from different manufacturer though. If not, then that way you can have more displays.
PCI Express x4 should be enough for GT520. No damage should come. Just wondering how you like to use those cards simultaneously. You cannot combine them into one faster card but with that configuration you can get more display connectors.
Four pieces? If yes, then that's only USB extender. Not hub as hub expands one USB wire to several connectors.
Markings on cable are probably general markings for USB type cable.
That hub may be OEM part, not sold separately so identifying it precisely will be very difficult.