Hello Florian
Question 1
The Root bridge never sends out TCNs because a TCN is used primarily to inform the root bridge of the topology change. More accurately, it is used to inform all switches between the topology change and the root bridge of the change. If a topology change occurs on the actual root bridge, it doesn’t need to send a TCN based on the above description. Note the following Cisco documentation:
Question 2
When a port changes from Designated port (forwarding state) to root port (also forwarding state) it doesn’t have to go through the process of listening/learning/forwarding. It is already in the forwarding state and it remains so until it goes down and is brought back up or until it receives superior BPDU due to a topology change.
I hope this has been helpful!
Laz