The record of Aron-Mottel's birth is transcribed here
http://usdine.free.fr/birthsjakobstadteightythree.htmlThe David Fellman who married Khaya Kortin in 1911 was probably a cousin of 'your' David who married Sadie.
It is not possible to determine the sequence of birth from the names that couples gave to their children. Jewish naming customs do, however, help to identify familial relationships, because a child is only ever given the name of a
deceased relative, such as its grandparents, great-grandparents, uncles/aunts, etc.
The name Gershon does not appear in any of the other Fellman lines in Jakobstadt or Riga, but it does appear in the Landman family of Jakobstadt. In particular, we have Marcus (Mottel in Yiddish) Landman (1816-1880), who was the son of a Gershon Landman. It is this fact that led me to deduce that Gesa-Eida was Marcus's daughter, because it would be logical for her to have sons named after her father and grandfather. As Marcus did not die until 1880, his name could not have been given to a son born in 1871.
Similarly, we can deduce that Leizer-Mendel's father, Leib, was still alive in 1871. His grandfather, Yankel, was most likely dead however. So it would be reasonable that the name Yankel be given to one of Leizer-Mendel's sons.
We can only speculate about the relative after whom Rebecca was named. As you say, she could have been Gershon Landman's wife.
Leib/Leyb/Layb/Laib/Leybel, etc. etc. are all
Yiddish variations of the same name derived from the Yiddish word for 'lion'.
However, you have to bear in mind that these are the romanised versions of names written in Yiddish (i.e. Hebrew letters) or Russian (Cyrillic).