If the neck is really unfinished, that would have been done after it was purchased, probably sanded down by someone who wanted to make it smoother and therefore faster. The finish protects the wood; so that when you remove the finish, you are removing the protection. This may have contributed to the twisting. If cost were no object, sending it back to the shop would be the best way to go. If there is no finish on the neck, a refinish at the shop is not inexpensive. A refinish and the neck work would probably cost more than a similar Essence in good condition would sell for on the used market. You may want to take it to a good local luthier and have them give an opinion on what they would do and what it would cost. It may be the case that, as David suggested, a refret and levelling may make this bass work at a fairly reasonable cost.