ServUO is a fork of RunUO. I believe version 57 is the last one that supports The Second Age (T2A) and older expansions, while version 58 supports Endless Journey (EJ) only. RunUO is the original project, which had stopped development for a long while but resumed a few months ago. RustUO is a new project by the RunUO team that aims to rewrite Ultima Online (UO) in the Rust programming language.
Yes, it's his project. You can check it out by going to the project's Discord.
Ranuo 3.1 is an outdated version of 2.3, simply ported to .NET 8. It lacks the changes added in Mark's RunUO 2.7.
RustUO appears to be the main project, but it's not yet released.