A few questions:
Does it matter that this is a very common area for accidents of this nature?
Does it matter that the officer asserted my guilt before ever asking me what happened?
Is the best option to try to plea down to following too closely or is there an even better plea to be had?
Will the prosecutor need all of the other witnesses (from the 3 other cars) to prove his/her case?
Will the prosecutor tell me if the other witnesses are there if I ask him/her?
2) Depends what you mean. Did the officer completely reject evidence to support his view? Or did he simply have grounds to believe you were at fault and acted accordingly. Keep in mind that once an officer forms grounds to believe you're at fault, they need to advise you of this. If not and you provide a statement, it would be rejected in Court.
3) Pretty much anything is better then careless in terms of insurance. If demerit points are a concern, fail to turn out to avoid collision is another option. I believe it only carries 2 points instead of 4.
4) I would say no. The driver of the vehicle you struck would be key, but less so for the others.
5) They should, but some people have had mixed experiences with Crown's.