Very impressed, will try to simplify some daily patterns with this.
What is the relationship or difference compared to screen or tmux?
Mprocs' purpose is to define multiple commands you usually have running (compilers, webpack, database) when working on a project and conveniently run them. I personally use mprocs inside of tmux window.
Same kinda question, but form pm2 or forever?
Mprocs lets you see outputs of processes (with tty support) and interact with them (for example, you can use vim inside of mprocs). In other words, pm2 is for asynchronous process management and mprocs for running a process in terminal but multiple at the same time.