Yes, you'll need to splice together the harness that comes with the radio to the harness that adapts it to your car. Since you're rolling with a DE, you won't have to worry about the steering wheel audio controls... that brings another level of pain in the rear to add an aftermarket radio. I personally recommend soldering the connections between the radio harness and the adaptor harness and using heat shrink on it, but that's just because I've done stereo install stuff for a long, long time.
And yes, you can reuse the dash kit and the stereo harness kit if you don't like the el cheapo deluxe radio but compared to the stock trash radio these came with, it'll be an improvement. The funny thing is, with today's technology, the dash and wiring kits will cost more than the radio itself hahaha.