many 486 motherboards have both a din connector and a ps2 connector for the keyboard. you can only use one. that would explain "kill the keyboard" and "irrelavent characters" plugging a mouse into a keyboard connector would do both. im assuming that your keyboard is in a din adapter or is a din style keyboard.
your only option may be to use an old serial mouse or keep looking till you find a ps2 to serial and ps2 mouse combo that works.
