Electronic – get 24V from ATX PSU using 12V and -12V cables

Yes. The current through the converter is determined by the load. I doubt the monitor is pulling all 5 amps from the original supply anyway. Just throw an ammeter on the ground of the pc side and measure it to be in the safe side, but it should work without modification. With 100% efficiency the input would be, at max draw, 12v 10A, for 24v 5A. In free air inside the pc case, with fans you should be alright assuming 4A load draw

Your PSU seems to have a single-rail 12V output. If you will keep the wires (yellow and black) to all your Molex 4-pin connectors of the same length (or if you plan to re-work the 12V connection to some other connector or directly to a board), you can reasonably assume that the current will be equally distributed, more or less, and you will have all 16A at the end with no problem.

If your PSU has multi-rail topology (has 12V1, 12V2, etc. labelled outputs), you might want to put small serial (equalizing) resistors (0.05 Ohms or less) between ganged outputs from these separate 12V1 and 12V2 etc. outputs, to accommodate possible voltage differences in the rail's outputs.