As you say the fan is triggered around 100C so at the point the cooling system should be fully functioning with the stat wide open (and the bottom hose too hot to hold).
However if your bottom hose was cool enough to hold then maybe its an indicator that the efficiency of the rad is overwhelming the stat and keeping it closed whilst the car is moving? That might be why you are seeing those high cylinder head temps as the bye pass coolant short circuits the rad.
Maybe try a stationary test for comparison? When the engine is cold start the car and let it idle until the fan kicks in at 100C. Then again feel the bottom hose. Is the cooling system now working normally with the stat open and the hose too hot to hold? If yes then the stat is perhaps not being overwhelmed here as the rad's delta T will be much less without the moving airflow passing through it.
