-- 1) Open Chrome.
-- 2) Find and Download APK. https://i.imgur.com/ZFZb1uE.png
-- 3) Accept warning and Open APK.
-- 4) Go to settings. (This only has to be done once) https://i.imgur.com/R8FzTzP.png
-- 5) Toggle Install Unknown Apps for Chrome. (This only has to be done once) https://i.imgur.com/K0ADO2q.png
-- 6) Click back (This only has to be done once)
-- 7) Click install. https://i.imgur.com/xVSndex.png
-- Done. https://i.imgur.com/fyasTK9.png
Once you do this for the first time, the process reduces down to 4 steps each time after: Open Chrome, Download APK, Open APK, Click Install. Done.
As someone who has worked retail in the past, and who currently works in IT: starting with this assumption generally minimizes headaches down the road.
There are only so many hours in the day: people learn what they need to get their job done, and tend to move on. Some people are quite dim, but others don't have the time/energy/motivation/need to learn the details.
About Aunt Susan , you could have the device locked by default and have a more complex process to root the device, like some code/password that is in the box of the device, in that envelope Apple PR team could inform Aunt Susan that she should not do this unless she is tech competent or a communist.