How to book a split stay

[pinkerbell]

we have booked the bounce back free dining for 14 nights, the maximum. We booked Friday Sept 26 - Fri Oct 10. I realized DD's birthday is our last day and want to stay throughtout that day and leave the next day Sat, maybe even sunday. We fly southwest and I think dings this time of year are grat for any day of the week. Do you agree? Also are there any good discounts on rooms for 2 extra nights (satying at ASM? And lastly what are the chances we can stay in the same room? I was told when I tried to add there are no guarantees, if we had to switich rooms after 15 days I would probably just arrive on Sat instead of Friday to extend the full day on DD birthday and not add any.
Thanks