Complex sentence: There exist some doubts to the validity of this treaty: the 13th Dalai Lama himself denied that he had authorized Dorjiev to negotiate a treaty with Mongolia and neither the cleric nor the Tibetan government ever ratified the treaty.
Split into simpler sentences: There exist some doubts to the validity of this treaty: the 13th Dalai Lama himself denied that he had authorized Dorjiev to negotiate a treaty with Mongolia. More importantly, neither the cleric nor the Tibetan government ever ratified the treaty.