1. a. is coming (a taxi does NOT have a schedule!)
b. comes (scheduled, so we use present SIMPLE)
2. a. is going to win (evidence, so pretty certain)
b. will win (a prediction without evidence)
3. a. will (for expressing hopes we often use will)
b. am going to see (intention)
4. a. am helping (an arrangement: present continuous)
b. might (it depends on indicates uncertainty; there is a condition)
5. a. am going to travel (intention)
b. am travelling (arrangements have already been made: I have the tickets)