Garmin explains the water rating http://www.garmin.com/en-GB/legal/waterrating of these devices as well as water rating definitions http://www.garmin.com/en-GB/legal/waterrating-definition.
Touch screen watches are not yet really suitable for aquatic activities, as contact with water generates false touches. In order to avoid this problem, it is recommended to create a shortcut to lock the screen when using Apnea.
Garmin does not want to publish applications such as freediving, diving, base jump, etc ...
Here a message from Garmin about that : App Aproval Exceptions
No, because it does not have a pressure sensor.
The new models (see non-exhaustive list below) refer to a maximum pressure value of ~1.7Bar, i.e. a maximum measurable depth of ~7m.
For more information, see the article https://www.apnea.me/blog/new-versions-apnea-dyn-1110-and-apnea-cwt-180
List of models limited by Garmin: Fenix 6, Forerunner 945. Not currently tested but supposedly limited: Venu, D2 Delta, Forerunner 645.
Apnea is available in English and French.
No, because third-party applications run in a virtual machine that completely isolates the application from the rest of the system.