The right thing evidently the wrong thing
After going to Walgreens and getting a syringe and mixing up some bread bits with water, the wildlife rehabber I managed to get a hold of last night told me she didn't think the egg would have even hatched if Mrs. Finch had really been gone as long as I thought, and told me to wait until morning and watch for her again. I've been watching for the last 45 minutes. I just checked the nest again. It's dead. The other eggs have not hatched, and I'm feeling pretty confident at this point they never will.