Thank you guys for all of your advice. It's such a hard decision with the girls for me, and it's one I haven't had to make yet. The boys I have no trouble dispatching but there's something about the hens that I just have a hard time with!
So now for an update on Harriet... I started the penicillin on Tuesday, she'll get her third shot today. I also tried to drain the swelling on Tuesday but poked in a several places and couldn't get anything. I decided to stop and try again the next day as I felt bad about poking around on her too much. When I checked her yesterday the swelling had gone down significantly and skin was pink rather than red. She was definitely feeling better and much more energetic. I gave her the second penicillin injection and moved her back outside to a hutch in her regular pen. This morning I checked her again and she was begging to get out of that hutch, running back and forth across the front. I pulled her out and the swelling is almost completely gone, skin is back to normal color rather than red. I put her back with the rest of the flock and she is acting totally normal - scratching, eating, running around with her sister. She's been getting steadily better since the penicillin shots started and I never was able to drain the fluid but it's seems to be working it's way out of her system. Droppings look normal, no sign of egg matter working it's way out or anything like that (not sure if I would see that or not with an internal layer?)
Is this the kind of upturn you guys were talking about previously and I should expect a downturn, or did I misdiagnose and it's likely some other type of infection rather than internal laying? I'm very curious to figure out what's going on but at this point I'm remaining cautiously optimistic.