Maximus,
You stated that William and Bonnie Heath bought the land that was to become Beth Sarim. Actually, they bought the land next to Seth Sarim, in 1939, and built Beth Shan on the adjoining property. The deed had William Boyd's name on it and he was holding the property in trust for the "ancient worthies" mentioned in Hebrews Chapter 11. The deed to Beth Sarim had Rutherford's name on it with the same trust arrangements. (signed in 1929)
William Heath worked for Coca Cola in Atlanta before going to Bethel. My mother referred to the Heath's as Coca Cola heirs, and so it may have been his money that paid for the mansion.
Beth Sarim was built in 1929. William Heath couldn't have been involved. He didn't become a Witness until 1932 and didn't meet Rutherford until 1934. William became Rutherford's private secretary in the summer of 1937. He spent the first two month's on the job on a two month speaking tour with Rutherford. It was on this tour that he met Bonnie. They were married the following January, one week after William's divorce from his previous wife. From then on, the Heath's were always with Rutherford on all trips until Rutherford's death.
The only other person that I can think of that has posted about these early years, was DannyBear. As I recall his parents worked on the WT farm on Staten Island, where Rutherford spent his weekends writing his books.