I'm not sure why you couldn't make the flow I describe just as accessible as anything else in Android? But I'll grant your premise and respond anyways.
If the user lacks full mental faculties, they are part of the userbase we need to protect from scams. Most likely, a user without full mental faculties who is trying to sideload will be a scam victim.
If the user lacks the necessary physical faculties to "solve a puzzle on their phone", they probably get help from friends regularly; a friend should be able to help with sideloading. Enabling sideloading should be a one-time operation right?
If the user lacks full mental faculties, they are part of the userbase we need to protect from scams. Most likely, a user without full mental faculties who is trying to sideload will be a scam victim.
If the user lacks the necessary physical faculties to "solve a puzzle on their phone", they probably get help from friends regularly; a friend should be able to help with sideloading. Enabling sideloading should be a one-time operation right?