I'd rather go the wired route, using the splitter in the attic. I can fish the wire myself and let them make the connections. Everything will be out of sight and easy to access later.
Just make sure there is a 110v outlet close by. You can either wire the cable into a jack and use the short Ethernet cable with the DECA or simply terminate the long CAT-5 cable and plug directly into the DECA. The tail on the DECA will connect to the power supply that comes with the kit and the installer will need to use a short coax cable to connect from the F-Connector on the DECA to your splitter. Then (assuming other end of Ethernet cable is connected to your router), you should see green lights on your DECA and be done.
What kind of temperatures do you see in the attic over the summer? It will probably be OK, but I'm not exactly sure what temp is "too high" for the DECAs.