I need advice about the logistics of getting to the airport.

1) What TAXI TELEPHONE NUMBER should I call for a BELMONT TAXI 
   to take me to Alewife, if that became necessary, with the
   intent of taking the Silver Line from South Station?

2) In that case, at what time should I telephone the taxi; 
   at what time should I ask to be picked up at 174 School St.?

3) Nathaniel has offered to drive me to the airport. Should I accept
   his offer? If so, at what time should we plan to leave Belmont, 
   a) in consideration of the heavy late afternoon traffic
   b) in consideration of Nathaniel's possible default, and my last
      minute need to call a taxi.

4) Should I ask Nathaniel to drive me to Alewife instead of to the
   airport?

5) Should I instead take the bus to the subway at Harvard Square?