Who served the longest in the Council of the Twelve without becoming president of the Church?
Here's answer:
Elder Franklin D. Richards, called at age 27 in February 1849, served until his death in December 1899 at age 78, a total of 50 years 10 months. However, he served as president of the Council of the Twelve for just a little more than one year.
Source: Deseret News 1995-96 Church Almanac, p. 52, 79.