The current launcher is made using Java 11 librairies named "JavaFX" and Minecraft 1.8.9 is using Java 8 thus requires both. Using the windows installer you can get easily your hands on Java 11 which is bundled into the installer! for linux & mac you will need to install it yourself here!
No, we support "Mojang/Microsoft" we don't advertise unpaid version of the game.
Yes you can use the Shadow Client to play on any Vanilla minecraft servers without issue!
Yes, all the additional items, enchantement, potions, gear & more are exclusive to the Shadow Network and won't be public.
To put it in simple terms the server has a life cycle in which after a determined timespan everything reset* and players starts fresh on a new map this is called a Wipe or new Version these often also improve/change the gameplay by adding new Items, Blocks, Enchantement, Potions, Animals/Monsters, Events and more
*Reset: Ranks bought from the Online store is kept and tags and achievement too!