It sounds reasonable to me, but I implore you to not allow these guys to just coast until the next deadline. You need to
1) get a full project plan, including all of the checking and sign offs that need to occur
2) follow up on the status of those items in that project plan
On November 26th, you said "While I can't give a hard date and say "absolutely" this is the date, it looks like the week of the 11th, but that's the "fuzzy" date I have at the moment and I'm waiting on confirmation on a not-fuzzy date from the foundry right now." This appears to be an 11th hour change to that early commitment and you need to be a lot more aggressive with them now on meeting the deadline. Even though that was fuzzy, they still are missing that date by a wide margin. As such you really do not have any reason to trust there new date.
I am telling you now that the future of your entire company probably rests on making this date. While upgrade customers are a little locked in at this time, if Avalon ships, then you can expect a large number of cancellations to follow.
@ Avalon, how that update going? Isn't it your time to shine?