Update: I SOLVED IT!
Hey, internet stranger 
If you got this far without a solution, here is what has worked for me (may need to install puttygen first with sudo apt-get install puttygen):
chmod 600 id_rsa
puttygen id_rsa -O private-openssh -o id_rsa.conv
This will convert a key into private Openssh format (even though it already should have been one). Source: Describes how to install and use puttygen on Linux. Puttygen is a command-line tool for generating and manipulating SSH keys for the Linux version of Putty.
This command will generate an id_rsa.conv and the generated file will already have the correct permissions (chmod 600). Ssh-ing with id_rsa.conv got me into Daniel’s desktop.
I don’t know if it’s a combination of things I’ve done previously, but id_rsa was a freshly copied and pasted key into a new file, I’ve nuked all my previous attempts and started over. I’ve also noticed that previously the very same command didn’t work, so it must be the fact it’s a new file.
Happy hacking!