So I found the check box.

Answers to your questions:
1. First of all I must complain: these are already four questions! ;)
Not in the car but I did in an experimental circuit to the battery and it worked as a warning for a forgotten seat belt. There is no 8 pin connector in my car. So I have to collect the wires seperately from behind the dash and use shoes [that's the Dutch name] to slide over the pins.
There is no belt buzzer in my car; there is an empty three pin connector where it used to be however. Some owner before me must have removed it. There is an S here [red-white indeed], a black/white cable coming from the door and a grey one [I presume from the light switch]. There is an S on the chime, yes.

2. I checked all wires with my multimeter in the car and S from the ignition switch doesn't give 12V. Well it does: when I push the key ... I guess there is play in the key or in the lock, maybe it has worn. Before I repair this, it's no use installing the chime. I don't know how to make it warn me for the headlights so you could be of help here.

3. The Volvo part number is: 1324061

Pins:

GF2 GF1 GF/L S
58 Su 15 31