Well, this keeps getting more complicated. NTSC vertical sync is defined as:
- pre-equalizing pulses (6 to start scanning odd lines, 5 to start scanning even lines)
- long-sync pulses (5 pulses)
- post-equalizing pulses (5 to start scanning odd lines, 4 to start scanning even lines)
Each pre- or post- equalizing pulse consists in half a scan line of black signal: 2µs at 0V, followed by 30µs at 0.3V.
Each long sync pulse consists in an equalizing pulse with timings inverted: 30µs at 0V, followed by 2µs at 0.3V.
This is obviously more than can be done with a simple sync summing circuit. I'm not sure how to do this without adding some rather complex circuitry. And I haven't found any commercial device that will do it.
But It may be that the monitor will sync without all those equalizing pulses. From your posted pictures I see that the vertical sync pulse is about 1ms which is roughly equal to the vertical blanking interval that normally contains all the vertical sync and equalizing pulses.
So the scheme would be to invert the horizontal pulse with a transistor inverter and sum that into one input of the 4 channel Maxim chip with the vertical sync. Then sum that with the video into the second channel.
So that may work, depending upon whether the monitor will sync properly on that long vertical sync pulse without the equalizing pulses. No guarantee.
It's your call as to whether it's worth the effort to try.