Read-Only or Snapshotted System For Elderly

These are likely advanced options. However, I can offer two possibilities.

As a previous poster mentioned, there is Guest Account. However, on logout, all data in the guest account is discarded, so this means there can be no persistent data maintained across a logout, reboot, or update. However, you can customize the User Template to an extent. Each time the guest account is logged in, the User Template is copied to /Users and anything that is in there becomes the foundation of the user's session. The User Template is located at /Library/User Template. Inside is a folder for many of the localized countries and another to all (Non_localized). You could create a user account, define all settings, copy them to the User Template, and now when using Guest, the initial settings are all in place. I don't think this will work with Keychain but it may work if you are using a 3rd party browser that maintains its own password database.

Ok, that will be a lot of work and a lot of troubleshooting to get it right. And in the end, it still may not be what you are looking for.

A second option is to look at a product like Deep Freeze. Once again, you will need to do a bunch of work to set the "perfect" starting point for the user. Once you have this in place, you "freeze" the device and now you can do anything you want to the machine and a simple reboot brings you back to the frozen state. We used this years ago for kiosks and public access devices that needed to be reset to a specific state. Now, you still run the risk of arbitrary data creation. If your relative is 100% using a web browser, that is one thing. But if they are creating local documents (screenshots, downloading images, writing a document, etc.) then you run the risk of that content being lost on reboot. You could use external media or the cloud for file storage, but that may be confusing for the user.

Hope this helps. I feel for your plight as dementia is a terrible scourge that slowly takes our loved ones from us, leaving behind a hollow shell of the person we know. May this help you find a solution.

If they are set up as standard users, not admin, they may mess with their own accounts, but not the system. However, even that can be cause for trouble and confusion.

You can certainly create many replicas of the same account (and I will tell you one way to do it), but there are issues with this, namely: 1) any documents created in one account will not be accessible in another; 2) if you set the login window to list of users, they will all be presented to the elderly person, which I am afraid may be more confusing; unless you set it as "user and password", but that too may be another hurdle (e.g reminding them to type a certain name followed by 1,2, or 3)

Here is one way to replicate an account: create and set up the account just as you want it; backup using Time Machine; migrate from backup - you will be asked if you want to replace or rename - in this case, rename. Repeat

Would using a Guest account work? Once the user logs out, it reverts to its original state. You don't have to give them access to anything you don't want them to have. You retain access to the admin account.

You may want to look into configuration profiles that business & schools utilize to further lock down a Standard user account. I'm not sure exactly what you can achieve with this, but perhaps you can prevent access to certain settings even within their own user account.

I'm not sure if there is any good "parenting" software which could impose some restrictions as well.

Maybe you can somehow use Carbon Copy Cloner or SuperDuper to replace some items within the home user folder, although you may need to script it to be more automatic. This may need to be performed from the admin user account depending on the severity of the corruption within their own Standard user account.

Is anyone in their neighborhood helping them out? Perhaps you can see if they would assist in "fixing" the account with minor intervention if you come up with a simple plan.

I don't think the guest account will let you save passwords for websites. It resets everything when the person logs out.

I've never used Screen Time to restrict someone else's access so I really don't know if it can do what you want.

Set up Screen Time for a child on Mac - Apple Support

How about an iPad with a keyboard case.

Less to mess up.

Those are some great suggestions. !