Lets try a different approach/asking this a little differently.
What is the simplest mod you can make for a forge server, using mixin? I.e. -- as an "hello world" type program to get started with?
I'm not sure I understood what you said about shadow not working right/not playing nice with liteloader.
The example project in the Forge MDK is essentially Hello World. On game startup, it prints some stuff about dirt blocks to the console.
And Sponge IS a replacement for spigot. The only thing about it is that for forge, it's required to be a mod. For vanilla, you just need to launch the jar as is.