That Russell inherited business interests from Joseph Russell is an overblown statement. Russell was partners with his dad in one store only. All the rest of his business interests were developed on his own or in partnership with others. Bet you didn't know he owned a furniture store? Oil and other interests too? And that he managed his father's used equipment and scrap iron business? His dad was worth almost nothing at his death. His dad's probate records are available. You'll find that his net worth at death was very small. Other than a contribution to the watch tower, what there was went to his widow. By the time J. L. Russell died in 1897, he had exhausted most of his money, some of it in poor investments.
View him as a crank if you wish. But a huge amount of what you find online is just contrived. Schulz and deVienne are the true exerts in this field. Contact them through their blog. The link to it can be found in an earlier post.
I haven't researched Rutherford enough to answer your question. However when I was in college the first time (decades ago. you probably weren't even born) the society's net worth could be accounted for by compounding interest on a six thousand dollar investment made in 1879. It didn't happen that way, of course. The Watch Tower Hemoraged money in the 1880s, most of it from Russell's own pocket. The claim that Conley financed the society is also overstated. If you saw the historical exhibit in Brooklyn, you would have noticed that his donation was 4000 US Dollars to Russell's initial 7000 USD. By 1882 Conley was already involved in the faith-cure movement and no longer contributing. Russell thought his translation was near in 1881, and he expended money as if it were water. He started selling his shirt and hat stores in 1884, selling them over time. But he continued to invest to raise money for the Society and on which to live. By 1896 or so, his net worth had declined some, but was fairly stable.
He invested in real estate in his own and in the society's name. He had oil interests and turpentine interests. He was defrauded twice by A. D. Jones who turned into a criminal of the worst sort, short of murder and mayhem. Russell invested in a furniture company, in oil drilling and extraction equipment, in typesetting equipment. Online somewhere is the claim he had an interest in a gold mine. The facts do not sustain the claim though there is a story behind it. The names of some of his business partners are fairly easy to find. He had some wall street investments, though they seem to have been small. (Sole source is the Russell v. Russell transcript). If Russell had invested the money derived from the sale of his stores (1884-1896) he would have died a wealth man. He put it all into the publication and circulation of WatchTower literature. He wasn't a charlatan, but he was convinced he had the truth. Sometimes those who believe fervently in something are the most deluded.