I'm afraid the answers to all your questions may not be as trivial as you hope.
The "DT830D" is made by dozens of eastern factories, as is the S by the way. Undoubtedly the origin of it is somewhere, actually using the prescribed intersil chip. But as in pictures it is already clear that the different available choices differ greatly between the different multimeters, I don't think anyone can make a unilateral promise that the die-chip is 100% compatible with any original intended chip.
If you really want to, it is probably possible to find out the original intended (probably also intersil) controller and the assumption that the signal pin is in the same spot on the knock-off meters is a reasonably safe one.
But then, do be aware that the layout of the front switch is very likely to be entirely different, so you will have to adapt the solution to take that into account.
The best way to find out which pin to use, probably, is just look at the PCB, identify the pins that seem like they are connected in similar ways, and measure them with another multimeter. The pin that gives a clear and fairly accurate 0mV to 200mV during a full scale voltage measurement is probably the one you want.
You can also scan all pins, for example set the multimeter to 0V to 2V scale, measure 2V (or a single cell battery at 1.5V) and see which pin has the original voltage divided by 10. There may be more by coincidence, but then measure those (probably not more than a couple) over more input voltages over the entire range as said before.
Another thing to be aware of is the fact that this trick is not very accurate. It's not a 1% or better PC multimeter when you're done. You are always going to get less accuracy than the device itself, but this one doesn't sport accurate voltage reference to measure against or high precision ADC, or low tolerance, low drift components, so the accuracy isn't going to be top-shelf. So don't expect to be making single millivolt measurements with reliability on the PC. The multimeter itself may drift a bit more than by design after mod, but shouldn't be too noticeable.