Im not the developer but I can answer some of these(and they can correct me if I’m mistaken).
$289 indie/professional license is a standalone perpetual seat license for that version(all minor updates). Only the enterpsie version requires a yearly renewal.
Standalone WC2 is indeed a unity program.
There is currently no C++ integration with UE4(or any integration). However the ways it’s implemented doesn’t prevent it from being used inside UE4, and it’s my belief their (eventual) intention is to make this into an engine plugin in the same way the unity plugin works; however that is separate from the stand alone version.