That is all somewhat very bad.
When I consider what I had when I myself was "pretty in the neckiest woods there are" for a while, then I assume they HAVE the possibility to:
1) Receive e-mails in an email-client that allows them to "click on things", e.g. start a HTTP download from somewhere, however proxied that might be
2) Receive attachments in that mail client that they can run.
Then I would suggest a Linux distro with a lightweight Window manger. I have put about a dozen of "Mail-And-Web" clicker-people on LXDE (I did it with Debian, but Lubuntu might be simpler to set up). Pretty simple desktop, with task bar at the bottom where you can put a start menu or quick launchers or other widgets.
I did this setup for a few relatives when they ditched XP (or even Win95) in the last years. Basically just removed the start menu, just put a "Mail" and "Web" button on one side, and the clock and the logout button on the other side of the task bar. Nothing else.
Then you could write a script that you put on the machine (in bash, perl, python, whatever you fancy), which executes scripts you send them with a special extension, and configure those scripts to be executed with your master-script when they click on the attachment. (You could add a key/hash in the comments of the scripts you send them, and check that with the master script you put on their box), and then mail the output of the script back to you.
You could also put all kinds of helper functions in the pre-installed master script, like download files from http sources, or restore the last backup, or create a system report and mail it to you, etc......