John William Kirtland married Sarah Elizabeth Comstock 8 Jan 1868 in Adrian, Lenawee, MI. John was my 1C4R and Sarah was my husband's 6C4R so we were especially interested in them. Sarah was the daughter of Addison J. Comstock and Sarah Swift Deane. There's a profile of John W. in Portrait and biographical album of Ionia and Montcalm Counties, Mich. : containing full page portraits and biographical sketches which is available on ancestry.com, or let me know and I'll send you the excerpt. The Comstocks were early settlers in Adrian and there's lots of information about them, especially Addison's father Darius Comstock, a notable Quaker and Erie Canal builder.
Addison L. Kirtland married Laura Stebbins 23 Jan 1891 in Lakeview, Montcalm, MI. This branch of the family went to CA and I think they're still out there.
I found the burials of the Slaters in Throopsville Cemetery, near Auburn, Cayuga, NY thanks to your info. George Cheever Jewell graduated from Yale in 1871 and was in Auburn with his mother Mary Ann Corwith Jewell Bradford and the orphaned Slaters in 1880 as you noted. I think George married Susan Elizabeth Wilder or maybe Draper 18 Sep 1874 in DeRuyter, Madison NY. I haven't been able to find her in 1880 but the family is in Highland, Union, Iowa in 1900. The census image is very faint. They are in Iowa in 1910 and Susan is widowed in 1920. I don't think either of their daughters married.
Isaac Slater married again after Angelina died and had another daughter Melita but then he died when she was 1 month old or so. His second wife Ella (later Wimble) applied for his Civil War pension but I haven't seen the file.
Maybe I can get out to the cemetery sometime and see if the Slater stones are there. I found Mary Ann C J B in Hector this summer.
Thanks for helping me fill in some of the gaps.