Your question below with answers:
Is it possible to drain most of the coolant by just draining from the lower drain plug in the radiator ?
I would guess that probably 50% or more of the total amount of coolant can be drained through the radiator plug. This will drain the entire radiator, tank, and as much of the block as is above the lower part of the water pump. Make sure you open the pressure cap before you start to drain.
If the only way for a complete drain is by also unplugging the block, I will do that as well.
Yes, the only way to drain the entire coolant contents is to drain the block. Even then, some coolant will still be in the heater hoses and core, but that is a relatively small amount.
I'm always confused about the role of the thermostat. Is the coolant entering the radiator from lower hose and exiting the top or vise versa ?
Vice versa. The coolant travels from the radiator to the water pump through the lower hose, and is then circulated through the engine block. Until the coolant reaches operating temperature (usually around 180°F), the thermostat remains closed and there is no circulation to the radiator through the upper hose. When the engine reaches operating temperature the thermostat begins to open allowing coolant to travel to the radiator via the upper hose. The thermostat modulates (varies its amount of opening) to try to maintain a relatively constant temperature of coolant in the engine.