Personally, I wouldn't recommend Arch (even the "friendly" variants like Antergos) to the less technically inclined. You don't need a to be extremely familiar with Linux to get started with Arch, but you do need to be comfortable at the command line and are expected to be able to read the (excellent) docs and debug things yourself.

It's a very educational experience, but not really something the "average grandmother" is likely to have the patience for, particularly when they're just trying to watch a flash video on Facebook. That's the real strength of Mint, and I'm not sure what best fills that niche if Mint is off the table.

Ubuntu with a post install script that grabs flash etc?

EDIT Write the post install script yourself don't use one of the many flawed ones that a search will show up.

I really don't see why the downvote? The OP wants an easy to use distro that their grandmother can use but seems happy to install it them-self.

flash? isn't that what the chrome cage is for?

Maybe the OP wants to use Firefox?

All of the Linux Mint application work well on Arch Linux (Cinnamon, Mate), too!

The documentation is good, I'm using it to configure KVM on Fedora now.

As an Arch user, the lack of MAC in any well supported form throughout Arch and its derivatives is a deal breaking turn off to me. I cannot imagine telling someone who is not technically literate to start using a Linux distro without being able to trust that arbitrary software cannot read all their documents and steal their passwords.

I have just installed Manjaro and find it to be fast, stable and easy to use.

Exactly the response I was about to give!