Coolant does not keep the temps down. Coolant (instead of plain water) prevents it from freezing and prevents corrosion.

Tight valves do not cause overheating. But they can cause the engine to die. After changing the head gasket, adjust the valves before driving it!

Where should the temperature gauge should be for a new fitted a15? In the dashboard or under the dash is the usual place. The temperature sender should go in the stock location, although it can also be fitted to the front or back of the cylinder head ( it should go into the cylinder head). Don't fit the sender to the radiator hoses, as you want to measure the engine temp, instead of the radiator temp. The radiator temp will go up and down as it is NOT regulated by the engine thermostat.