Instead of using fabric's events, a kind of hook interface is better.

src/main/java/net/pugsworth/manythings/ManyThingsClientMod.java

@@ -4,17 +4,13 @@
 import net.fabricmc.api.EnvType;
 import net.fabricmc.api.Environment;
 import net.fabricmc.fabric.api.client.render.EntityRendererRegistry;
-import net.minecraft.client.MinecraftClient;
 import net.minecraft.client.render.entity.ArrowEntityRenderer;
 import net.minecraft.client.render.entity.FlyingItemEntityRenderer;
-import net.minecraft.entity.Entity;
-import net.minecraft.util.ActionResult;
-import net.pugsworth.manythings.client.EntityCreationStruct;
+import net.minecraft.entity.EntityType;
 import net.pugsworth.manythings.client.event.ClientEntitySpawnCallback;
 import net.pugsworth.manythings.entity.ModEntities;
 import net.pugsworth.manythings.entity.projectile.ArrowExplodingEntity;
 import net.pugsworth.manythings.entity.thrown.ThrownDynamiteStickEntity;
-import net.pugsworth.manythings.mixin.client.ClientPlayNetworkHandlerMixin;
 
 @Environment(EnvType.CLIENT)
 public class ManyThingsClientMod implements ClientModInitializer
@@ -25,21 +21,21 @@ public void onInitializeClient()
         EntityRendererRegistry.INSTANCE.register(ArrowExplodingEntity.class, (dispatcher, context) -> new ArrowEntityRenderer(dispatcher));
         EntityRendererRegistry.INSTANCE.register(ThrownDynamiteStickEntity.class, (dispatcher, context) -> new FlyingItemEntityRenderer<ThrownDynamiteStickEntity>(dispatcher, context.getItemRenderer(), 1.0f));
 
+        ClientEntitySpawnCallback.HOOKS.add((packet, player, world) -> {
 
-
-        ClientEntitySpawnCallback.EVENT.register(
-            (world, player, packet) -> {
-
-                if (packet.getEntityTypeId() == ModEntities.THROWN_DYNAMITE_STICK_ENTITY)
-                {
-                    Entity entity = new ThrownDynamiteStickEntity(world, player);
-                    EntityCreationStruct struct = new EntityCreationStruct(entity, packet);
-                    struct.createEntity(world);
-                }
-
-                return ActionResult.PASS;
+            EntityType<?> id = packet.getEntityTypeId();
+
+            if (id == ModEntities.THROWN_DYNAMITE_STICK_ENTITY)
+            {
+                return new ThrownDynamiteStickEntity(world, player); 
             }
-        );
+            else if (id == ModEntities.ARROW_EXPLODING_ENTITY)
+            {
+                return new ArrowExplodingEntity(world, player);
+            }
+
+            return null;
+        });
     }
 
 }

src/main/java/net/pugsworth/manythings/client/event/ClientEntitySpawnCallback.java

@@ -1,26 +1,15 @@
 package net.pugsworth.manythings.client.event;
 
-import net.fabricmc.fabric.api.event.Event;
-import net.fabricmc.fabric.api.event.EventFactory;
+import java.util.ArrayList;
+
 import net.minecraft.client.network.ClientPlayerEntity;
 import net.minecraft.client.network.packet.EntitySpawnS2CPacket;
 import net.minecraft.client.world.ClientWorld;
-import net.minecraft.util.ActionResult;
-
-public interface ClientEntitySpawnCallback {
-    Event<ClientEntitySpawnCallback> EVENT = EventFactory.createArrayBacked(ClientEntitySpawnCallback.class,
-			(listeners) -> (world, client, packet) -> {
-				for (ClientEntitySpawnCallback event : listeners) {
-					ActionResult result = event.onEntitySpawn(world, client, packet);
-
-					if (result != ActionResult.PASS) {
-						return result;
-					}
-				}
-
-				return ActionResult.PASS;
-			}
-	);
+import net.minecraft.entity.Entity;
 
-    ActionResult onEntitySpawn(ClientWorld world, ClientPlayerEntity client, EntitySpawnS2CPacket packet);
+public class ClientEntitySpawnCallback {
+	public static final ArrayList<ClientEntitySpawnHook> HOOKS = new ArrayList<>();
+	public interface ClientEntitySpawnHook {
+		public Entity getEntity(EntitySpawnS2CPacket packet, ClientPlayerEntity player, ClientWorld world);
+	}
 }

src/main/java/net/pugsworth/manythings/mixin/client/ClientPlayNetworkHandlerMixin.java

@@ -1,23 +1,18 @@
 package net.pugsworth.manythings.mixin.client;
 
-import java.sql.Struct;
-
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Shadow;
-import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 
 import net.minecraft.client.MinecraftClient;
 import net.minecraft.client.network.ClientPlayNetworkHandler;
 import net.minecraft.client.network.packet.EntitySpawnS2CPacket;
 import net.minecraft.client.world.ClientWorld;
 import net.minecraft.entity.Entity;
-import net.minecraft.entity.EntityType;
-import net.minecraft.util.ActionResult;
-import net.minecraft.util.math.Vec3d;
-import net.minecraft.world.World;
 import net.pugsworth.manythings.client.event.ClientEntitySpawnCallback;
+import net.pugsworth.manythings.client.event.ClientEntitySpawnCallback.ClientEntitySpawnHook;
 
 @Mixin(ClientPlayNetworkHandler.class)
 public class ClientPlayNetworkHandlerMixin {
@@ -29,12 +24,17 @@ public class ClientPlayNetworkHandlerMixin {
 
     @Inject(method="onEntitySpawn",at=@At("TAIL"))
     public void onEntitySpawn(EntitySpawnS2CPacket packet, CallbackInfo info) {
-        EntityType<?> etypeid = packet.getEntityTypeId();
-
-        ActionResult result = ClientEntitySpawnCallback.EVENT.invoker().onEntitySpawn(world, client.player, packet);
-
-        if (result == ActionResult.FAIL) {
-            info.cancel();
+        for (ClientEntitySpawnHook hook : ClientEntitySpawnCallback.HOOKS) {
+            Entity entity = hook.getEntity(packet, client.player, world);
+
+            if (entity != null) {
+                entity.updateTrackedPosition(packet.getX(), packet.getY(), packet.getZ());
+                entity.pitch = (float)(packet.getPitch() * 360) / 256.0F;
+                entity.yaw = (float)(packet.getYaw() * 360) / 256.0F;
+                entity.setEntityId(packet.getId());
+                entity.setUuid(packet.getUuid());
+                world.addEntity(packet.getId(), entity);
+            }
         }
     }
 }