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I am not sure if source code can be verified.

Knox is closed source so it can't be verified. All AirGap product is open source and free. We went this route because we want this to be a paid product and anyone with the code can easily reproduce it.

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Can AirGap Knox be installed on any Android devices, included devices with custom ROM's or Pixel phones that operate with GrapheneOS (not that I would do that), or stock Android must be used?

Nope, Knox is meant for Android enterprise devices. which comprises over 550+ devices as of today.

https://androidenterprisepartners.withgoogle.com/devices/#!?device_categories=knowledge_worker

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Correct me if I am wrong but I think we can achieve something similar to disable everything on software level, and for little more money we can already buy brand new hardware wallets.
One more question... can AirGap Knox be removed from device and how?

Hardware security models aswell as the security design in general of mobile devices is way more secure and battle tested compared to the currently available hardware wallets.

The reason is, these devices don't have the privilege of "not care" about the physical attack vector, yet everyone wants to use them them for very security relevant tasks (mobile banking, identity management, etc)

The knox lockdown performed is one step deeper than what is offered to the user through i.e. settings. Also user errors (i.e. accidentally turning on wifi) are taken care of by the lock down.