Is healing from envy only achieved by the envier's ablution and bathing with his water, or is reciting the Two Protectors (Al-Mu'awwidhatayn) and other invocations sufficient for protection and healing, especially when maintaining them without a noticeable change in condition?

If you predominantly believe that the envy stems from a specific person, it is permissible for you to ask them to perform ablution (ghusl) for you, though this is not obligatory. However, if you do ask them, they are obliged to comply, due to the apparent meaning of the Prophetic command: "And if you are asked to perform ablution (ghusl) for someone, then do so." The person who casts the evil eye is commanded to perform ablution and is compelled to do so if they refuse.