Blower runs continuously unless I switch off at Heater (ON/OFF) switch. From
Thermostat (Pro) I switch from Heat to OFF mode and blower runs forever.

There could be any number of causes. If the fan is controlled by temperature, the fan switch could be stuck or bad. If it is timed, you could have a stuck fan relay on the board. If there is a manual fan switch on the furnace, as there is on many Colemans, it could be turned on.