I doubt a Dell Optiplex will support raid functions. Kind of useless anyway with SSDs, very rarely would you ever use that much data throughput and the DMI to the CPU would likely get saturated anyway.
I don't have the cable in front of me, so I can't tell you if they have added circuity to produce 5V, it is possible, a simple (and terrible) resister divider could do it. Or a ligthweight DC-DC buck converter.
There are two basic types of Dell. Stock ATX and OEM Proprietary.
A stock system with a normal 24-pin ATX power connector will have SATA power cables coming directly from the power supply. There you would attach the splitter if there aren't enough connectors.
Dell's proprietary OEM 12V only standard will have a 4-pin and 8-pin power 12V connectors to the motherboard. The motherboard will provide 12V, 5V, and 3.3V and have a SATA power cable plugged into the motherboard. You would be able to attach the splitter there.