the actual entity class please
I don't see where preInitRegistries() is being called.
You should probably try registering your entity renders like this.
https://github.com/Cadiboo/WIPTechAlpha/blob/fc508e4dd439603888ca2caf08c0b871372764d4/src/main/java/cadiboo/wiptech/EventSubscriber.java#L325-L332
If the method is static you don't need an instance to invoke it. This is basic java.
As long as RegistryHandler is subscribed to the event bus this will work, yes.
This is not how vectors work. You need to get an angle between your vector and the default unitZ vector to get the rotation you need to apply around the Y axis.
As for your direct issue - you must bind the TextureMap.LOCATION_BLOCKS_TEXTURE before rendering your TE otherwise the last bound texture is used.
The simplest way would be to open Powershell with the right-click method, then type in cmd and press enter, this will swap to a command prompt while using the same window and staying pointed at the same folder, then use the command given in the other thread.
Open notepad or another basic text editor, and type
cmd.exe
into the file.
Save the file into the same folder as the jar file, but save it as .bat instead of .txt - this will make a runnable file that will be in the folder you want.
Then you should be able to run the command.
Step 1:
Step 2: