Joanne, please write off-line at my e-mail address (neumannATsasktel.net-- change AT to @). I have a long reply with findings on Carl and Caroline (Manthey) GROSS, and not all are interested in the details-- census records, death certificates, naturalization. The main finding is that they emigrated in 1888 from Mogilno, Posen (south of Bromberg)-- they are actually listed on the same page as some of my relatives-- Neumann-Kettner relations. You mention Julius Manthey dying in Flatow, and I found Auguste Gross listed nearby Carl and Caroline, and she was from Flatow also.
I also found a few possible brothers for Caroline in the Chicago area.
This should be of interest to you. All the best,