.....Using the AutoPilot requires that you have the optional (but free) pump control relay, and gives you both freeze protection and failure to prime protection.
BEEN READING THE AUTOPILOT MANUAL. THE FREEZE PROTECTION WORKS ONLY ON SINGLE SPEED OPERATION
AND DOES IT BY MEASURING WATER OF 40° AS THE TRIGGER...DOESENT SOUND TO GOOD TO ME.
Using your existing timer has the advantage of being less work to wire up and not requiring the AutoPilot relay.
For high speed/low speed control I would just use the manual switches already on the pump, with it programmed to default to low speed and use high speed as a manual over-ride when you need it.
As mentioned previously, it is very likely you have a good bit more than 15 GPM on low speed, but the only way to be sure is to try it.