I have seen copy of 1850 census page for John K. Dow of Washington, Lincoln County Maine.It has John K Dow 55 M, Hannah 53 F, John 25 M, Ensign 23 M, Madison 21 M, Leander 19 M, Alfred 17 M. All listed as b. Maine and all the males listed as farmers.
There is a separate census record for Warren, Lincoln Co. ME that lists a Madison Dow 21 M farmer and a Leander Dow 19 M farmer living at the home of Edwin Andrews.I would expect that is a duplicate reporting when census was taken - especially since these were adults and living in a non-Dow household.
Although this isn't proof that it is the same Leander and John K. you have listed, at least it shows a John K had a son named Leander.Book of Dow gives birth as 1831 and d. 1915 - but no source, although some information on this line was provided by Leander A. Dow b. 1886 and grandson of Leander b. 1833. The younger Leander was the son of a John K Dow whose wife was a member of Mass. Society of Mayflower descendants. I don't know if her interest in her own genealogy would have included any research into her husband's line, but that is a possible avenue of research.
I can provide copies of the census pages if you need them. Please email me directly for that.