Beta 5 is not open source, but the -outdated, Emral's fork should be enough to around with the editor.
As for the official guide, it doesn't work unfortunately (at least for Windows, I haven't on other OS). Yes you need CMake +3.5, 7Zip and Git, but you need a custom toolchain for QT and MinGW.
Here are the versions you need of
QT and MinGW
MinGW and
Ninja.
- QT goes in C:/Qt/
- MinGW in C:/mingw-w64/
- Ninja in C:/mingw-w64/mingw64/bin
Then follow the guide as usual.
I use the 38A package because it's the most lightweight.
As for your other questions, I think you're better off with going at WohIstand's server to familiarize yourself with the code. Have fun!