lordsuperyahweh: DMM's parent company owns a few dozen studios.
For your second question check out the latest post at
https://www.akiba-online.com/threads/setting-a-jav-company-in-japan-possible.1595023/page-2
Profit margins are a good question and it really varies from studio to studio and title to title for both western and Japanese companies so it's hard to answer.
Most top tier AV stars are not 'whoring on the side'. Some did do brothel work first, but you'd be surprised at how many were just regular women with zero adult experience before deciding on doing AV.
IMO, the richest people in this industry are the CEO's of the modeling agencies and also the CEO's of big online marketplaces too. Famous AV stars make a lot of money and hopefully most don't waste it, but that's an individual issue.
Anyway, your last comment about 'living on your back' was a bit harsh, no? Not all of them are stupid. Far from it actually.