After a while on constant tinkering, it seems modding in custom arrows is rather difficult.

 

When it comes to entityTrackers. they have their issues, bugs and seem to never get the same result as an arrow. The only way to properly get this to work well is extending EntityArrow.

 

but by extending EntityArrow, it seems it's only half the required support for custom arrows. many required fields are private. and only damage and texture are available. this means the item picked up, the gravity, and other various effects can not be done. and adding custom scripts over will ruin the tracker and scripting of the arrow. Adding getGravity(), getDroppedItem(), and even onEntityHit() methods would make arrows more customisable and well fit for easy implementation.

No, you can subclass and override functions in there already. If you need access to something transform it in your dev env.

it seems that works fine in creative, but the entityTracker messes the angles when fired in survival.. so i think it might be either a bug or something else.

Sounds like you're not registering your custom entity properly, as as long as you pass in all the information that you need instead of relying on the arrow's internal functionality. There are many mods that add projectiles. Go look into those.

well i can't imagine survival mode/creative mode having a different outcome, and there's only the register and entity tracker that's needed. (and to note, other mods with projectiles also have issues with entity tracking)