I just got a second MacBook Pro for work-related, so I have a 2018 560 and a 2018 Vega 20. To answer your questions:
1. It came with Mojave 10.14.1. It's actually a slightly newer build of Mojave 10.14.1, 18B3094, likely updated for Vega support.
2. Since the 2018 versions came out I cannot get "Setup from Time machine" to work. It just says there is only 1.4 MB of data to bring across, there is some sort of security issue. Some have had no issues, maybe it's because I have the disks encrypted - but when I tried to go from a 2017 to a 2018, I even tried un-encrypting the disk and setting up from Time Machine, with no luck. What I did then, and I did now, was update "From another Mac". I got a cable to go Thunderwire Mac-to-Mac, and it was really fast and worked perfectly - it only took about 30 minutes for a 1.5 TB installation to migrate across. DO NOT try this via Wifi, after about 20 minutes it said "405 hours to go" lol.
Hope that helps!