I haven't looked much into the JK converter, I'll assume the 3rd wire is a ground. The one I posted does not connect the ground from the USB to the BMS, I don't think this would matter, maybe someone could confirm, either way it would share a ground somewhere else in the system.
Yes I would still need a "tiny connector" on the BMS side, this looks like a standard PCB style molex socket.
Yes the USB dongle will accept the signal straight from the BMS (although untested yet) that is essentially what it's designed to do, and very likely the same as what's inside the JK converter..
The only difference I see with the JK converter is, it looks like it takes its power from the BMS rather than the output, I think this would mean full battery voltage so probably has some sort of step down converter inside.