It would be better for traffic control if trainz AI did not control the switches but just made the trains obey  the signals in a similar manner found in older game called Dispatcher.
   
  The way it seems to work is No 2 or No 1 signals allow the AI to find the path to the scripted destination. No 4 signals control only permission to enter next track section.    I have not observed that the No 3 signal has any more AI relevance than a No 4.   In a two finger branch then there would be three signals, a No 2 on the trunk and two No 4, one on each branch.  A three finger branch would have a No 1 signal and three No 4, one on each branch.
 
  Once the path has been determined by the AI using the No 1 and 2 signals, it then throws the switches.  As to how many switches it may throw along the path may contain the solution to the problem, if it is an absolute number,  one could build stub sidings to block long distance switching by the AI.   We know it must see the entire path before it will begin traveling to the next destination.  If you place a switch on a track with no corresponding branching of the track the AI is blocked from finding its way even though you could drive the train to that destination.
 
 When two trains seek the same switch I suspect that the one who gets control is the assigned driver closest to the beginning of the alphabet.
As to having too many signals in control of a switch it should make no difference for the signals do not control the switches as far as I can tell.  Of course, what we think we know often blocks us from finding the truth of anything, so the more light that can be shed on this, the better for all.
If my game behaves similar to others than you will see occasions where the AI controlled train will overlook orders in timely manner, be they trackmarks, destinations, or loading points, going right past where they should have stopped then waking up that they made an error and creating chaos as they try to reverse back to where they should have gone.  While the AI no longer throws switches under passing trains they still can run into each other, but when they do, they do not forget their orders when uncoupled like in previous versions.