How can one respond to a person who follows the Tijaniyya Tariqa and cites as proof of the correctness of his methodology that he performed Salatul-Istikhara (prayer for guidance) and saw a vision confirming it, and that his condition has improved for the better since following this tariqa, and he feels tranquility and reassurance with their litanies, and sees no shirk (polytheism) or ghuluw (exaggeration/extremism) in that?

The clarity of the answer to your question lies in understanding the fundamentals and branches of Islam, its sources of reception, and its methods of derivation. After the death of the Prophet, peace be upon him, rulings of Islamic law are received only from the transmitted narration of the infallible revelation (the Book and the Sunnah). Dreams are not a source for Islamic rulings, and it is not possible to believe that saints meet the Prophet, peace be upon him, while awake and receive directly from him.

Religion is not about whims, claims, personal tastes, or ecstatic experiences; rather, it is about adhering to the Book of Allah, to the truthful narration from the Messenger of Allah, peace be upon him, and to following the footsteps of the noble Companions.

As for the points mentioned in the question:

1. Salat al-Istikhara (prayer for guidance) is performed only for permissible matters. Visions seen after Istikhara are not to be relied upon for action or abstention. Such visions may even be from devils who deceive people, as explained by Shaykh al-Islam Ibn Taymiyyah. The statement, "Allah Almighty cannot place His servant on the wrong path after seeking His help and performing Istikhara," is a claim for which there is no proof, for supplication has conditions and impediments.

2. Frequent remembrance (dhikr), certain types of worship, and improved conduct with people are not evidence of being on the right path. Indeed, the devil may increase the diligence of a person involved in innovation to deceive him and others. The criterion is conformity with the legal evidence from the Book and the Sunnah. Action without emulation only distances its doer from Allah, and innovation is worse than sin.

3. The constant recitation by the Tijaniyyah of "Salat al-Fatih" and other innovated supplications holds no weight, regardless of their alleged positive effect, as they are rejected innovations.

4. Good intention and pure motive do not benefit their owner if they stray from the true path. Many who engage in misguided deeds think they are doing good, while they are in manifest error, due to their turning away from the remembrance of Allah and the devil beautifying falsehood for them.