Is it necessary for the Yanyu Jianghu Bagua Sect to gain full favorability?

No need. According to the query of relevant public information: In the game "Misty Rivers and Lakes Baguamen", if the favorability reaches 27,000, you can purchase the Yin and Yang Feng Shui plate and give it to Dong Sanchuan. It is learned that it can be used in Huashan, but before using it, you must first find an old carpenter in Tai'an Town. Repair yin and yang feng shui. No need to fill it up.