laptop motherboard "shorts" when connected to adapter

Is there any battery installed in the system (CMOS or regular laptop)? I had an Epson laptop with an external battery that would behave that way (the LED would): If you plugged it in it would light up, but as soon as you tried to turn it on, the light would die. I suspect in your case, the battery is discharging slightly through the cable when it is connected.

The spark when connecting the adapter is more-or-less normal for a capacitive load. However, the fact that the voltage goes to zero would indicate a damaged adapter, a short in the internal power cable, or a shorted mainboard.

You reportedly tried with a different adapter, so it's probably not the adapter. (Did you check the on-board voltage with the alternate adapter?) You can connect the internal power cable to the adapter but leave it disconnected from the board, then meter, to see if the internal power cable seems OK.

But if the adapter and cable check out it's sounding a lot like a bad mainboard. The disk drive or DVD drive could possibly cause the short, but if you've tested with those disconnected (and also presumably looked for obvious external shorts between metal components and the mainboard bottom) then the mainboard is strongly suspected.

I think you gave it the old college try with this, but one isn't going your way.