1) It is to cool the IC that allows you to use 4x NVMe M.2's.
2) You shouldn't need a motherboard/chipset to perform bifurcation since that IC will already take care of that.
That one from Amazon only supports SATA M.2's, not NVMe M.2's.
Try using a 24V to 12V 10A step-down DC-DC converter (you will need to do your own wiring of the plugs and some basic electronics knowledge) and attach it to your existing power bank while it is outputting 20v.
Something like this...