IT was still hackable on the earlier mavic models (up to Mavic 2 Pro i believe) and yes it was very useful.
I often used it to judge actual wind at altitude (take it up, put it on ATTI and you can clearly see the exact wind direction and speed...
Not quite.
If they're jamming GNSS the drone will drop down to atti mode and you fly it that way.
If they jam control as well then it'll drift off on the wind.
4g won't help. That's easily jammed.
I hacked my Mavic 2 to reallocate sport mode to ATTI. Yep its fun and fast and also useful in areas with weak GPS that would cause spirals and drift.
I dont think its possible on later drones, its not disabling sensors, its telling it to ignore...