What is your highest clock frequency for your PCB?

The following is my answers for your questions: 1.Doesn't matter. You just need a different trace width depending on which you choose. 2. What is the optimal trace width for high-speed digital signals? There isn't one. You can choose a width to have a given impedance. 3. Do I need any termination for 5-10cm traces with 1-3ns fronts? At 1 ns and 10 cm, yes. At the weavelength is 30 cm, 20 cm at electrical speeds. 10 cm is almost half that, well above the guidelines.

For your question about thickness,the optimal thickness would be less than 0.010" between your signal layers and your power/gnd planes. But since you have a 2 layer PCB there is no point in worrying about PCB thickness. The thickness will be whatever is required to give the board the mechanical strength required to hold all of your parts, about 0.0625".