When you ground a19 it's almost as bad as applying vcc to it.
I would simpely take a resistor and put it in between the switch and a19/18., this way both currents are limited.
As to trying to find the correct value, it would probably be trial and error, try a relatively big one like 100 Ohms first in both situations (a18/19 to ground or vcc), if the switch doesn't work anymore or the x-box chrashes just lower the resistance.
At some time you wil have found a value that everyting seems to work, take about 70% of that and that would be perfect.
(so if it's a 10ohms resistor you found to work, you should use a 7ohms resistor)
Always try to find the maximun resistance first, how higher the resistance that everything works the better(minus the safety ofcource)
At this time i do not have the resources to try this, but i would not do it any other way(as an official techie)
If anyone tries let me know, in the meantime i wil try to think of a more educated guess.
There are also small heatsinks like those on pc motherboard cihpsets, these couldn't hurt also because overcurrent means heat and heat means dead components, so that's a second way to secure your x-box life
That one then goes on the big chip without a cooler on it.
