There was a lot of trouble communicating the contents of the R-15 version with the people that okay the games due to a language barrier, as well as a registration fee of $100. I don't remember if there were any other issues with it. This is what Nana shared with me when I was asking about the Steam sales when the game was released. Nana has said repeatedly over the course of developing the R-18 version that they plan to release on Steam, but getting the discount for owning R-15 doesn't seem to be possible, and he isn't sure that he'll be able to get it on there at all due to the H content. It may simply just take longer to get the Steam version out than DLsite. I'll ask about it.
About purchasing points on DLsite, this is just how I do it, but I click on the points, go to "Purchase Points" on the left side, select payment with a debit card, enter the card information, and confirm the transaction. Should be as simple as that. 1 yen = 1 penny = 1 point, disregarding exchange rates. A purchase of 3,000 points (roughly $30) will be enough to cover the game's cost.
Well, I got beaten to the punch on this, but I'll post it anyway. If about $1,000 in sales is necessary to make a profit, then yeah the R-15 version barely went past that. That's awful then. Hard to say whether or not the R-18 version will even sell more than the R-15 version on Steam, it's not exactly a platform made for H-games.
EDIT: Asked Nana, he said he does plan on applying for Steam, but the application process may take longer than DLsite, and he isn't sure if he'll be able to bundle the R-15 version with the R-18 version.