You are right that you will have difficulty getting ADRs at many restaurants at this late date -- especially the Epcot restaurants because of the CP packages. Many of those restaurants not only won't have ADRs available, they won't be taking any walkups either.
You can also expect similar difficulties at MK venues. When we were there in December two years ago, there was zero TS availability anywhere in MK for the entire 5 days we were there. We had our ADRs, but many people were disappointed.
You should have better luck eating in resort restaurants. You'll probably be able to get ADRs
somewhere, but possibly not your favorites. I'd try to make the ADRs first, and if you can't get anything decent, just skip
DDP. It's really not worth it unless you're getting full value for the TS credits.
Generally, TS credits are not exchangeable for CS meals. A couple of years ago, because of the popularity of free DDP, they allowed the "trade-in" of TS credits for CS and some bonuses. But they haven't done that since.
Like anything else Disney, your results may vary from restaurant to restaurant, and even from server to server. Good luck!