Do you (or does anyone) have any ideas how this could have happened -- this could potentially really help other users not face the same issue.
That's easy, don't do this:
Mistakes made- Depositing before downloading, saving and verifying the Letter of Guarantee
- Sending a large amount of Bitcoin to a relatively small and unknown service
- Sending your entire Bitcoin savings at once to a third party
- Keeping your life savings entirely in Bitcoin
Malware (did a malware search)
You should never trust a malware search for 100%. There's always the possibility of unknown malware.
Cloned site (Was using PureVPN and incognito mode, very cautious of typing in the correct url: mycryptomixer.com)
I'm pretty sure a VPN server can do the same thing
Tor exit nodes do.
Tor exit nodes (Was using Google Chrome on the main site, not onion)
As far as I understand, .onion sites can't be altered the way "normal" sites can be altered by a compromised Tor exit node.