At least it's good to know that there was no negative voltage :)
As for the extra sensor pin, you could use one of the five analog input pins A0-A4 on the 14-pin extension header. But you have to add a 10uF capacitor to ground and a 4k7 resistor to 5V yourself (see "Thermistor" section of the schematics on the link in my first message). You could e.g use a 0.1" 2x2pin connector and bring out A4, PWM, GND and 5V to a little prototype PCB where you can implement the thermistor RC circuit and the sevenswitch for the heatbed.
As for the "3 things" you did, it looks to me that no. 2 could probably be the cause of the problems - shorting with the PS case (sparks!) may have damaged some parts of the Sanguinololu.
As for the extra sensor pin, you could use one of the five analog input pins A0-A4 on the 14-pin extension header. But you have to add a 10uF capacitor to ground and a 4k7 resistor to 5V yourself (see "Thermistor" section of the schematics on the link in my first message). You could e.g use a 0.1" 2x2pin connector and bring out A4, PWM, GND and 5V to a little prototype PCB where you can implement the thermistor RC circuit and the sevenswitch for the heatbed.
As for the "3 things" you did, it looks to me that no. 2 could probably be the cause of the problems - shorting with the PS case (sparks!) may have damaged some parts of the Sanguinololu.