Continuing due to image limits...
Feferi was not into him. It's not clear if she once was and his behavior has destroyed the relationship or if it were always a matter of obligation, but it quickly becomes clear that her initial conversation with Kanaya is more putting a brave face on a bad situation than anything. It's likely that their relationship would have ended around now even without Sgrub. It's nigh impossible for them to be happy moirails in future fic without there being a major event to cause the change of heart.
Eridan indeed got the rifle when Vriska got the dice. I'm not sure if we ever had confirmation of when that was, though.
He may possibly have been the one to retrieve them both, given they were apparently at the bottom of the ocean and I don't think we've seen any suggestion it's possible for land trolls to go underwater. Air tanks are certainly well within their technological grasp, but they may just not make them. That said, the way he talks does make it sound like they were both there getting the stuff together. Considering Alternia has actual roving zombies, it may well have actual ghost ships that emerge from the depths.
One of the interesting bits! Eridan admits that he's commissioning them only as an excuse to be in cahoots. He also expresses, in the same paragraph, irritation that they don't work. So did he want to use them or not?
From the whole thing, it sounds like he's saying he's continuing to commission them despite how useless they are as an excuse to stay in contact with Vriska when otherwise he'd have stopped bothering. Vriska figuring it out in that case would mean she didn't need to bother making functional machines because he'd pay her regardless. Still, calling the entire thing an elaborate ruse suggests that the devices were never much of a priority, and the fact he's putting this much energy into rekindling a relationship with her means he's probably unwilling to blow up all land dwellers in one go at the moment.