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Minecraft doesn't let you control how many levels an enchantment consumes, it's always 1, 2 or 3 depending on which slot of the enchantment table it's in. To add an enchantment, create a class that extends Enchantment and register it like any other IForgeRegistryEntry (i.e. IForgeRegistry#register in RegistryEvent.Register<Enchantment> or GameRegistry.register in preInit). Override Enchantment#getMinEnchantability and Enchantment#getMaxEnchantability to control the enchantablility and XP levels required to apply the enchantment. Create an EnchantmentType with EnumHelper.addEnchantmentType or override Enchantment#canApply/Enchantment#canApplyAtEnchantingTable to control which items the enchantment can be applied to. Override Item#canApplyAtEnchantingTable to control which enchantments can be applied to an item.

After changing the > to >= in your code, it correctly dropped a spawn egg in 1.11.2; though the spawn egg didn't have its entity set. I'd recommend using Entity#setDead rather than World#removeEntity. I'd recommend using the entity name NBT tag rather than using the global ID as the metadata, since this is what every version since 1.9 uses. In 1.8.x, set the entity_name tag to the entity's name (EntityList.getEntityString). In 1.9+, set the EntityTag tag to a compound tag with the id tag set to the entity's name (EntityList.getEntityString in 1.9-1.10.2, EntityList.getKey in 1.11+).

There is no Player class in Minecraft, you probably imported the one from the IXBM library. As @Kriptikz suggested, EntityPlayer is the class used by Minecraft to represent player entities.

instanceof EntityPlayer

Item#canApplyAtEnchantingTable doesn't seem to exist?

What you need to do is in your processInteract method when you call openGui instead of passing an x position you should pass getEntityID(). Then when you construct your Container and Gui pass World#getEntityBy/FromID(x)

When an egg damages an entity, DamageSource#getDamageType will return "thrown", DamageSource#getSourceOfDamage will return an instance of EntityEgg and DamageSource#getEntity will return the entity that threw the egg (if any).

This is an intended feature, not a bug.