The statements of Thakura Bhaktivinode are as good as scriptures because he is liberated person. Generally the spiritual master comes from the group of such eternal associates of the Lord; but anyone who follows the principles of such ever liberated persons is as good as one in the above mentioned group. The gurus from nature's study are accepted as such on the principle that an elevated person in Krishna Consciousness does not accept anyone as disciple, but he accepts everyone as expansion of his guru. That is very high position, called Maha-bhagavata. Just like Radharani, sometimes thinks a subordinate of hers as her teacher, to understand devotion of Krishna. A person who is liberated acharya and guru cannot commit any mistake, but there are persons who are less qualified or not liberated, but still can act as guru and acharya by strictly following the disciplic succession. It is the injunction of the sastras that anyone who sees the Deity in the Temple as made of wood or stone, or considers the acaryas and gurus as ordinary common men, and discriminates Vaisnavas or devotees as belonging to a certain group or caste, are called hellish. Your question about Bon Maharaja in relation with his disciple is very intelligent and intricate, and we shall discuss at long when we meet.
Regarding Kirtanananda's letter, I may inform you that I always think of him and pray to Krishna for his good sense. That is my duty. Anyone who comes to me for my help or wants advance in Krishna Consciousness, and whom I initiate and accept as my disciple, I must pray for him and his welfare always. So Kirtanananda personally served me, especially during my illness, which I always remember. But because somehow or other he has misunderstood our activities that does not mean that I am no longer his well wisher. I write at the end of my letters to my disciples, "Your ever well-wisher", and as such, I cannot become otherwise than being ever well-wisher of my disciples, even though he may leave me. So I was praying to Krishna that He may save Kirtanananda from his misunderstanding and if ever he chanted Hare Krishna at least once in sincere heart, I am sure Krishna would not allow him to go out of his influence. Therefore, I believe that he can never forget the Form of Krishna, neither he can deny His Personality. It is good news that he is trying to establish a new Vrindaban, which I suggested through Hayagriva Brahmacari, and if he is successful, in his attempt, certainly it will be considered a great benediction upon him by Lord Krishna. When I offered him Sannyas, I expected such great achievements through him and if Krishna desires, he will come out successful in his great attempt. Yes, I expressed my desire to go there through Hayagriva Brahmacari, and if I am invited, to go there, by Kirtanananda Swami, it will be my great pleasure to see the place and enjoy his company.