Suggestions

I've gotten the latest recommended version of the forge, but when I double click it to open it, it wont open. I'm on windows 10 but using java minecraft. 

Okay, so I removed MrCrayfish Vehicle Mod, and that fixed it. I didn't have to re-add the mod I removed.

You're not returning an instance, you're returning a holder to that instance. For that, you need to create a holder that references your instance. You'll need to use LazyOptional::of and pass the instance in and then cast it within the method. Note that you should always return the super of the method in case something else decides to override it.

That's not how biomes are handled anymore. You need to apply it via BiomeLoadingEvent. Also, the register to WorldGenRegistries is a bit pointless as you are attaching it to the biome directly after it has been created.

I know how linear interpolation works, but you seem to have not read my entire post. If the movement of the entity is global, then the lerping should will happen already from the entity movement. However, if it's happening relative to the model, then that's where you would need to do you own manual interpolation. Else, you are just interpolating what's already has been interpolated. You can view this within BoatRenderer. There is no interpolation for the movement, just the rotation of the model pieces.