> The hapless Android user does not understand the negative consequences of sideloading.

Then make sideloading disabled by default but enable it when the users tap 7 times on whatever settings item. At that time, explain those "negative consequences" to them, explain them real good, don't spare anything and if they still hit "Yes, continue to enable sideloading" you do that immediately in order to avoid increasing their haplessness with other made-up excuses.

Simple.