You can switch off Bluetooth on the mobile. The watch is only synchronised with the mobile if you switch on Bluetooth. Unfortunately, I don't know if it also works if you have Bluetooth switched off for more than 24 hours. Maybe someone else here can help you.
As far as I know, there is only a connection during synchronisation. The synchronisation has to be started on the vivofit, so only during sync a bluetooth connection exists. (This is also necessary to save battery, which needs to be changed only once a year.)
I have always worn all Vivofit products (versions 2,3,4) while swimming. So there shouldn't be any problems with the Aquafilt.
I take off the Vivofit (and also the waterproof wristwatch) for showering, as soapy water can penetrate more easily due to the reduced surface tension.
According to Garmin support, the device does not have this function, although it is mentioned in the Garmin specifications. However, it is possible that an update will be released soon that will enable the function.
Unfortunately, this question is very difficult to answer. The manufacturer Garmin does not provide any information regarding the component composition of its products, but points out that international and EU directives (RoHs directives and EU Chemicals Regulation (REACH)) are complied with. Furthermore, the products are CE-certified.
The technical support staff member I had on the phone said that he does not believe that the Vivofit 4 wristbands contain nickel.
However, I have read reports on various devices in the Garmin forum that an allergic reaction occurred when wearing the wristbands.
Why do you want to...? It has hardly any buttons and has to be operated somehow.
Synchronisation and activation are done via WiFi. There is no button to switch off Bluetooth.
If you do it with an app, how do you turn it back on?