The measurement of the number of floors is very accurate. For the steps, there is a deviation of about 1%, which is also quite accurate. I found this and more information at: https://www.fitbit.com/de/charge2 https://www.sagtdermeister.de/sport-freizeit/fitness-armband/#messung-und-genauigkeit found.

fitbit hard reset, see google for instructions, re-establish bluetooth connection

The Fitbit can only be used in conjunction with a smartphone with the corresponding app via Bluetooth. In addition, you need a Fitbit account. The Fitbit does have a built-in watch, alarm clock and other settings. However, these can only be synchronised with an app.

Yes, but you have to configure it explicitly. And only one variant is possible, e.g. SMS *or* Whatsapp (or a few others, Threema unfortunately not).