1 - Can I shoot several photos on the same roll with a different ISO setting on the camera and then see which one comes out looking the best? (Or do I have to shoot entire rolls at different ISO settings?)
You can and you should shoot test shots using several different exposure settings - simply keep track of them on a notepad. Set the camera on a tripod at a decently lit object and start at ISO 12 (it's old film, it probably got slower so start low). Personally, I'd shoot manual - get the first frame to a good exposure by way of the meter and then modify shutter speeds from there. For example:
- Frame 1, metered for ISO 12: 4s, f/5.6
- Frame 2 would then be: 2s, f/5.6 (assumed ISO of 24/25)
- Frame 3: 1s, f/5.6 (assumed ISO of 50)
- Frame 4: 1/2s, f/5.6 (assumed ISO of 100)
- Frame 5: 1/4s, f/5.6 (assumed ISO of 200)
- Frame 6: 1/8s, f/5.6 (assumed ISO of 400)
- Frame 7: 1/15s, f/5.6 (assumed ISO of 800)
- Frame 8: 1/30s, f/5.6 (assumed ISO of 1600)
And so on and so on. By decreasing the shutter speed, you are effectively treating the frame as if it's ISO were twice that of the previous frame.
When you've developed the roll, see which frame has the best exposure. This frame is the ISO you would then use to shoot other rolls. (Make sure that you are choosing a developer and sticking with it for these rolls unless you have an absolute ton of rolls to add that extra variable into the mix)
2 - Is there a way to "guess" whether it's color or black and white just by trying different settings in the camera?
No, the camera isn't going to know what type of film you've loaded. If the cannister had a DX code - then maybe. Your other question made it look like there wasn't one.
Other point: If it's C-41, it'd have an obvious orange backing. I don't know what C-22 looked like but it was in use during the era and so it is possible that your film is C-22. Both would develop in B&W chemistry.