Margaret - I know you and everyone else is concerned. The Watchers have been quiet lately because we've been trying to figure out what was happening. We didn't want to report until we were fairly certain that it was Archer.
I can't answer all your questions about the viability of the egg and history of this happening before. Ei has posted quite a bit about this over the last couple days. I'm sure she has more information to share.

Every situation is different.
There are quite a few potential nest sites at KP. Both Falcons have been seen at different spots, including the ORB, an Orange Railing Bldg that they seem to like. There are other spots that might be suited to raising Peregrine young. No copulation has been witnessed between Archer and Lady Pefa. They do a lot of flying around and I've actually seen them tandem hunt. He seems really interested in food when he's there. We have heard them ee-chupping.
We will definitely keep Watching!
Now another question: have there been instances when nests/eggs have been abandoned in such cases as this where the male visits two nest sites? I know that there have been instances where males support two nest sites, but A has been scarce at NB lately. At least it seems that way to me and I keep an eye on the NB througout the day.
All of these questions are popping up -- how long can egg sit as this one is -- un-brooded before it is not viable? Is there a time frame?
Is there a viable nesting spot at KP? I think a possible site was mentioned in a post this week.
Is there any chance KP security would cooperate with watchers to allow a closer watch?
Sorry for all the questions, but after watching our Beauty all winter -- survive, and thrive, and have fun with Mr. T, and the return of Archer , this is an unexpected turn.