Sometimes one actually does find what one is seeking. One of the first prominent workers was a man named Tavender. His name appears at least three times in Zion's Watch Tower, but never with a first name.
His name was Joshua Tavender. His obituary appears in The Utica, New York, Weekly Herald of October 15, 1895, on page 12. It mentions his connection with Charles T. Russell, the author of Millennial Dawn.
I am in urgent need of the first name of Miss C. B. Downing, the first Watch Tower missionary in China.
I NO LONGER NEED THE FIRST NAME. HER FULL NAME WAS CALISTA B. DOWNING.
Here is her history as I have it:
A letter from Chefoo (now Yantai), China, was printed in the May 1883 issue. Miss C. B. Downing, the missionary who wrote to Russell wasn’t the one to whom an issue of Zion’s Watch Tower was mailed. Instead, it was shown her “as a curiosity.” She read it carefully and with interest, explaining that he (or she) was “somewhat out of the orthodox ruts”:
If you will send me the paper I will try and get the subscription to you in some way--for, though a self-supporting missionary, I cannot quite call myself one of the "Lord's poor" to whom you offer the paper gratuitously, for Our Father has bountifully supplied all my needs, since I gave up my salary, three years ago. I think I can get a few subscribers among my friends in China, for I find not a few who are trying to reconcile the “mercy that endureth forever” with the final irrevocable doom of all who, since the fall, have died without a knowledge of the Redeemer of the world. We have no "Post-Office Order" arrangements here, else I would send the subscription at once. 
Her name isn’t associated with the letter; as was most often the case the letter was published without signature. But, in 1900 another missionary, Horace A. Randle, recalled:
There has been in China for years one solitary witness for the present truth, Miss Downing, of Chefoo. This lady was formerly a missionary of the Presbyterian Board and she chanced to meet with a stray Watch Tower, about the year 1883, in which she read an article on restitution, and at once decided to subscribe for the paper. 
C. B. Downing was viewed as a bit odd by other missionaries. “Amongst the missionaries of Shantung I am afraid Sister Downing was considered a queer old lady having some odd notions,” Randall wrote.
As with many of the early Watch Tower readers, finding biographical information on Miss Downing is difficult. A Miss C. D. Downing appears in the 1850 Census as a resident of Boston. That Miss Downing was born about 1825. I cannot state with certainty that she is the same as the missionary teacher in China.
C. D. Downing was a school teacher in Red Wing, Minnesota, before becoming a missionary, and as a missionary was supported with contributions from Westminster Presbyterian Church in Minneapolis.  She arrived in China in 1866 as part of the American Presbyterian Mission, to found a girl’s boarding school at Chefoo which she did by the next year. She continued to run until sometime before 1896. 
Downing participated in The General Conference of Missionaries in China. She was a delegate to their convention held in Shanghai May 10-24, 1877. She was associated with C. W. Mateer’s mission in Tung-Chow (now Tongzhou) and assigned to the station at Chefoo at least by 1871 and was still there at the time of the conference. The mission at Chefoo, “the chief foreign port of the province of Shantung” was established in 1862, the year after the mission in Tunchow. 
China wasn’t the United States. China’s population lived in abject poverty and superstition was rampant. It was heart wrenching. Probably seeing conditions in China as they were in the mid to late 19th Century had some influence on her ready acceptance of the message of the Millennial Restitution, the restoration of an Edenic earth.
Writing to the journal Woman’s Work for Woman in 1872 she recounted some of the heart-breaking and difficult situations she met: “In my visits from home to home I see many girls growing up in sin and ignorance whom I long to get, but their heathen relatives would ‘rather they starve’ than let them come to use. Many times they reject our offers to train their girls in our school, and sell them for slaves or for worse than slaves. Poor ignorant people. They will not believe we will keep our word with them, but think we want their girls ‘to take to foreign countries or to make medicine of them.’” 
Two years later, another letter from Miss Downing addressed the issue of child prostitution and slavery as she encountered it. The letter was addressed to a group that “had undertaken the support of a child in her school.” She wrote: “This little girl was a slave bought from a bad woman who had become ill and sold this child to get money to buy medicine. I do not know, nor does she, what her father’s name was. … I have another little slave girl who is very pretty. Of her parents we know nothing.” 
By 1894 at least she was no longer associating with the American Presbyterian mission in China. The Directory & Chronicle for China, Japan, Corea, Indo-China, Strait Settlements, ect. for that year lists her as independent.  She moved from principal of the girl’s boarding school to the staff of Temple Hill Chinese College in Chefoo. The educational directory that lists her as on staff says: “This school is not directly under mission control. It is self supporting. The strong religious character of the school and the establishment of similar schools in the city have somewhat retarded its growth.” With a Mrs. W. C. Booth, Downing was one of two foreign teachers. There were also six Chinese instructors. 
Though her most obvious missionary work was loaning Watch Tower publications and discussing the message of the impending Restitution of All Things with European and American missionaries, it is certain that her message went to her students too. A contemporary publication, The Encyclopedia of Missions, said of the boy’s and girl’s boarding schools at Chefoo: “Many have been received into the church who became interested in Christianity through what they heard from the children in these schools.”  So while it is true as observed by Carolyn Wah, that Watch Tower missionary activity in Asia “did not start among the Asians, but among foreign missionaries,” the tendency of C. B. Downing’s activity was to reach her Chinese students.  Even if her contemporary missionaries and teachers saw her as a bit odd, The China Mission Handbook reported that under her care, “the school has been a great blessing to our work.” 
Still, her primary mission field using publications was among English speaking missionaries. Writing to Maria Frances Russell in 1887 she said: “I am giving away and lending my copies of Millennial Dawn and my papers, and any time you can send me extra copies of the Watch Tower I can use them to advantage. I expect to see a good many missionaries from other parts of the country during the summer, as this is a health resort, and I shall scatter my Towers, and lend Millennial Dawns. The last bound copy I gave away before taking the wrapper off.” 
Still later, in 1888, she explained her work more fully:
The Dawns reached me on the 23d of September, for which many thanks. Three of the books are now in Shanghai. The good and thoroughly orthodox Methodist sister, to whom I gave one, said, "The restitution theology is very interesting, and I am glad you have found such rest and peace in believing it." I am sure she will read the book carefully, and be benefited by it. Another book has gone into a Baptist family. And the third I gave to Rev. Dr. W., who believes in the Millennial coming of Christ, and is, I think, somewhat prepared for Dawn. One book has gone to Ching-chew-fu into the Eng. Bap. Mission. The others I shall send--one to Peking, one to Amoy, one to Tang-chon, etc. The papers also arrived in due time and will soon be scattered over China. The books ordered came by last mail, received two or three days since. Since writing the above, the Concordance and Diaglott came. I cannot thank you enough for the kind letter received at the same time. I am using my Dawn, and the others and the papers are being scattered broadcast over the land. The Rev. Bp. S. has a Dawn. You may be sure I lose no opportunity to tell the glad tidings.14
1 View from the Tower, Zion’s Watch Tower, May 1883, page 1.
2 Randal, Horace A: Present Truth in the Far East, Zion’s Watch Tower, May 1, 1900, page 150.
3 School teacher: Rasmussen, C. A.: History of Red Wing, Minnesota, 1933, page 217. Church support: Fifth Annual Report of the Woman’s Presbyterian Missions of the North-West, Chicago, 1876, page 92.
4 The China Mission Handbook: First Issue, American Presbyterian Mission Press, Shanghai, 1896, page 199. Arthur H. Smith: Rex Christus: An Outline Study of China, The Macmillan Co., New York, 1904, page 112.
5 Records of the General Conference of the Protestant Missionaries of China, Held at Shanghai, May 10-24, 1877, Presbyterian Mission Press, Shanghai, 1878, pages 2, 5. Survey of Missions of the Board, The Foreign Missionary of the Presbyterian Church, January 1871, page 203.
6 Woman’s Work for Woman, September 1872, as quoted in Margaret E. Burton: The Education of Women in China, Fleming H. Revell Company, pages 45-46.
7 Woman’s Work for Woman, January 1874, as quoted in Margaret E. Burton: The Education of Women in China, Fleming H. Revell Company, pages 50-51.
8 Hong Kong, The Daily Press, 1894 edition, page 100.
9 Nathaniel Gist Gee: The Educational Directory for China, Second Issue, Education Association of China, 1905, page 22.
10 Bliss, Edwin Munsel, editor: The Encyclopedia of Missions, Funk and Wagnalls Company, New York, 1891, Volume 2, page 252.
11 Wah, Carolyn R.: Jehovah’s Witnesses and the Empire of the Sun: A Clash of Faith and Religion During World War II, Journal of Church and State, January 1, 2002. The article contains several errors of fact. She identifies William T. Ellis, a noted opponent of Russell’s, as a Watch Tower representative. She dates missionary activity outside the United States to “as early as 1892,” at least eleven years after it began.
12 The China Mission Handbook: First Issue, American Presbyterian Mission Press, Shanghai, 1896, page 199.
13 C.B.D.: A China Missionary Writes, Zion’s Watch Tower, October 1887, page 2.
14 C.B.D.: The Truth in China, Zion’s Watch Tower, February 1888, page 2.