Since posting these messages a few months back I have become aware of some additional sources which shed new light on this information.
Of particular importance was the discovery that many people of the Schönsee, West Prussia area belonged to the Ladekopp congregation.This led me to the Ladekopp Kirchenbook (church book) which features the following details:
An Abraham Hiebert was born on September 3, 1790 in Schönsee.Apparently the name of his father was unknown to the person recording this information, but no Hiebert other than Johann or Jacob from Schönsee is mentioned in this time frame.This date of birth would work for the Abraham Huebert from Halbstadt in the 1808 Russian census, as well as the Abraham Johann Hiebert from Halbstadt in the 1835 census.Also note that in nearly all these entries, the last name is spelled "Hiebert," rather than "Hübert" or "Huebert."
This church book gives the date of April 20, 1781 for the marriage of Johann Hiebert and Catharina Martens.It also gives the dates of birth of Johann and Catharina's younger children (in Schönsee): twins Jacob and Margaretha born December 13, 1795; and Maria born December 9, 1798.
This book also gives the date of death for the wife of Jacob Hiebert of Schönsee, at age 73, on June 13, 1796.This, of course, suggests that Agnetha Penner Wiebe, Jacob's wife at the time he immigrated to the Molotschna region of Russia in 1803, is not the mother of Johann Hiebert (whom I had previously thought to be born about 1760).The 1772-73 West Prussian Land Register confirms this, as it shows Jacob's wife to be named Catharina.This record, as well as a new book by Peter Rempel on the "Mennonite Migrations to Russia, 1788-1828," also help us fix the approximate years of birth for members of this family.Jacob would be born about 1726; his wife, Catharina, would be born about 1723.Their son, Johann, known as "Hans" at the age of 11, would likely have been born about 1761.The Land Register also shows that Johann had a sister, Catharina, born about 1756.
Again, if anyone would like any more information on these sources, please let me know.