Volvo XC60 D5 overheat

[Schalk007]

My 2010 XC60 D5 had a small water leak in the top water pipe for some time, i could not find it until about a month ago when it got big enough to notice the water dripping from the pipe. Finally been fixed, i took it for a test drive before filling it with antifreeze. I pushed the D5 a bit on the highway but when i turned off on a smaller road an drove much slower my water level indicator came on telling me that the water is low. I stopped next to the road and open the bonnet to check. I felt the top water pipe and noticed there was a lot of pressure in the pipe. Both the fans were on but very slow. I opened the water reservoir slowly and the water started to come out, boiling out... I filled it with water again while it was running up to its mark and closed the reservoir. I was not ever halfway though my test so i switched on my heater inside the car with all my windows open to help cool the water down. I drove normally home again.
At home i checked the pressure and water level again and everything was fine.
Here is my 5 questions: 1)This model do not have an engine temp indicator on the cluster (Can i put another cluster in that can show this?) 2) Why did my fans turn so slow at the time the water was boiling? 3) Where is the thermostat relocated, it was changed 60000km ago with a dealer, can the thermostat be faulty already? 4) If i plug out the sensor close by the starter at the water hoses my fans starting to run nice and fast, but then a message on the cluster saying 'cooling sistem needs service' so i plugged it back in again and the fans switch off again. 5) Could this sensor be faulty?
The Volvo is AWD and a Geartronic and the radiator is clean inside out.