If your planned system is in place, whoever has the phone with the app, has access.
My phone can pay for stuff at eftpos terminals with my thumbprint. Someone can steal it and try and use it, but without my thumb, it's a big ask.
No system is going to be perfect. In the same way that firearms laws only make life hard for law abiding firearms owners, and no inconvenience to a criminal, airside access is only a pain in the a$$ for people that already have a right and reason to be there.
If someone is determined enough, they are going to get in, and do what they want, and nothing short of a shoot on sight policy for anyone crossing the fence will stop that.
As for reception, I haven't been to a place that has security gates and ARO's/security but no phone reception in ... well, it's never happened, and I go to a LOT of remote places in Australia.
Airside security/access is a terrible, expensive joke at the expense of pilots, crew and the travelling public, and the sooner it all gets wiped and a system based on common sense gets implemented, the better. But enough of what I would do on my first day of my reign as a benevolent(ish) supreme ruler of the universe...