I guess I managed to break too many things while testing several settings...so I did a fresh Linux Mint installation and decided to try Crates option again almost succeeding...
Now I managed to perform cargo install mollysocket as well as create vapid key (though only as a plaintext..) and add that to corresponding file. I set up Mollysocket to be used in air-gapped mode (webserver = false), then performing systemctl restart mollysocket, and now based on systemctl status mollysocket the process is now active. I also got the QR code via journalctl -u mollysocket, but now the status on the Molly app says it's "Unable to create the linked device".
That's where I'm stuck now. Seems to be so close to work..
Yeah, someone just made a post about that recently. This solved the issue, and now I've my Mollysocket server running! Many thanks to you for assisting.