How can you 'regrow' the root nerve and pulp of a tooth after a reckless dentist has removed them in preparation for an unnecessary root canal?

The nerve can not regenerate in a tooth, instead what is left of it will eventually die. You would be better off having the root canal treatment completed and if it is a back tooth you NEED a crown on the tooth to prevent breaking the tooth in the future. The tissue inside a tooth cannot be regrown after root canal therapy. But you can keep your tooth all your life if the root canal is well done and well restaured, and if you brush your teeth well.

In the future, they may be able to harvest your own stem cells and implant the cells into your jawbone. This would heal or possibly regrow dead or missing teeth.