This is what I recall from some forgotten book I read some time:
1. Yes, below that speed all three rudders steered the boat.
2. Yes, above that speed the outer ones were fixed at those angles and the larger rudder was used for steering.
3. Yes, sure, the problem (if you´re trying to replicate it in your model) would be the scale you would need. I´m not sure if even a 1:24 model would be large enough (that is because the water has the same properties but your boat´s power will be scaled down).
4. Beats me…
I´d certainly like to read more replies to this thread!
Cheers.