Or can my coins be returned to me? I have all the evidence + PGP SIGNATURE
No, there is no way to retrieve them back even you provide any evidence. It is not the same as PayPal that you can dispute the transaction.
If it was sent last 2020-02-28 and confirmed more than 1+ there's no way to recover them unless if your transaction still unconfirmed.
Can you post the transaction ID here so that we can check if you still have a chance to get your BTC back.
BECAUSE THE SITE: bestbitcoinmixers.com IS A SCAM! SAME As coinmixer.es - SCAM!
Now, I see why you've been scam because of the blog post/article from bestbitcoinmixers with 5/5 ratings and good reviews which is a lie.
Next time don't trust any blogs/articles from Google they are sometimes indexed fake articles and rank up on the SERP. So next go directly here on the forum if you don't know if the website related to crypto is safe.