The dining plan is only an alternate method of paying for your meals. Use of a dining plan (regular or deluxe) doesn't exempt users from the cancellation fees imposed starting October 26. Dining plan users still need to take those fees into account when booking the affected restaurants. (However, it should be pointed out that CRT, HDDR and SOA have been prepaid meals since long before this year; they are not technically part of the "new" policies.)
The way they handle the prepayment is: if you have a dining plan on your reservation, they can book you with only a credit card hold, not full prepayment. You will, however, pay the full amount of the meal (not just a $10 no-show fee) if you don't cancel or show up for the reservation. As for partial parties showing up, we have nothing but conflicting information from Disney so far as to how that will work. Therefore I don't really know the current best way to handle your one adult who is undecided. If you think they will decide longer than 2 days prior to the reservation, you can probably remove them from the reservation by calling WDW-DINE. If you get someone who says they can't remove them, call them back and ask someone else.
If you are trying to book online and have entered your resort reservation number, and the system still wants you to pay in full (don't complete the transaction unless you're absolutely sure you are not being charged anything) then make the reservation by phone.