The high limit usually only affects the burners, shutting them off while the blower continues to run, cooling the heat exchanger to an acceptable level whereupon the high limit closes reigniting the burners.
The 2 relays govern the Fan On, (G wire connection) and the heating logic. The fact that replacing both boards with similar ones had no affect on the blower is most strange. For some reason there is some signal causing the blower to be powered. With the power off, I would disconnect all thermostat wires from the air handler control board. If the blower still runs, then disconnect all wires coming from the heat pump control board, totally isolating the motor and air handler control board.. We need to isolate the source of the wrong logic signal, if there is one. Check all wire connections for anything suspicious.
The other possibility is there is something wrong with the motor itself. That is why I wanted to determine the voltages at the control board where the heating and cooling motor wires connect.