Do you sell replacement fans for this unit?
I am sorry , we currently do not sell the fan separately.
Is the cooler slim enough and does not prevent the use of PI hats?
Yes, it does not interfere with the use of Pi HATs.
If we want a Kit that includes: Microbit v2 with Temp and Humidity sensor. Can these 2 sensors be directly connected to the Microbit or will need a development board?
these 2 sensors can not be directly connected to the Microbit , and you will need a base shield to connect them