Well my understanding is that Freddie greatly loved the Platters’ “The Great Pretender” that came out as far back as ‘55 and it was released on the label “Mercury Record!” Freddie often times would get his inspiration from other great singers/actresses (Liza Minnelli, Shirley Bassey, Aretha Franklin, et. al.) and incorporate what he’d felt were their best attributes to his stage persona, style of belting out songs, & overall artistry. Just like Freddie often was inspired by or ‘borrowed’ from other greats like Nijinsky’s character from his famous “Afternoon of a Faun” ballet where Freddie heavily borrowed the middle portion of the “I Want to Break Free” video sequence, he likely admired the Mercury Records label and kept their name in the back of his mind & revisit it later...when it suited him just like he’d later covered the Platters’ song “Great Pretender.”
Sofaras the ‘Melina Mercuri’ nickname, THAT was an inside joke between Freddie and Elton John. Elton suggested Freddie utilize a different name when booking a stay in a hotel to trick the press/paparazzi. He came-up with ‘Melina’ and Freddie came-up with the ‘Mercuri’ ‘cos he was a HUGE fan of Golden era Hollywood pictures and so the fun started with Freddie renaming just ‘bout all of his closest male friends some female names (Elton became Sharon, Rod Stewart became Phyllis, Bri became Maggie May, Peter Freestone became Feebie (now mostly known as Phoebe), Rog was Liz Taylor, etc. Mary Austin was named Steve Austin after the “Six Million Dollar Man” show main character.
So Freddie Pluto then... :-)