I'm not sure what you mean. If you already have a package booked that includes you and your aunt, there should be absolutely not problem putting your mother on the reservation.
No, they will not let you add a second room under your pin, because that will require a second reservation number. Your mother can book now and when the fall free dining offer comes out in a couple of weeks, she will just need to call and apply the code to her reservation.
Yes, that is correct, you could have booked 3 rooms under that pin code. You can not book another room under that pin now, because it is expired. But, you can add additional people on to your current reservation and discount with no problem.
They will announce free dining the end of March/first of April and when the code comes out all your parents have to do is call and have the FD code applied to their reservation.
There's not need to "change" the reservation, all the OP will have to do is call WDW reservations when the code comes out and have them apply the discount to their current package reservation. The official fall free dining dates have not been announced yet. They usually come out at the end of March/first of April. The pin code dates and the bounceback dates are for August 15 - October 2, 2010, so those are the speculative dates for the fall offer.
The fall free dining promo is always announced in the early spring. The dates are usually for mid to late August through the end of September/beginning of October.
The 35% or 40% room discount for deluxe resorts is pretty much always a better discount than the free dining discount.
