Thank you all for your help & advice. It's Wed night and Fifi the bantam hen is the contented mother of 4 fluffy baby ducklings. I am certain they would not have hatched without intervention.
There was one egg that never pipped at all, I'll leave it under the hen overnight. But I didn't hear any pecking or peeping from inside it. There was a tiny mark near the small end of the egg that might have been an attempt at a pip, I don't know.
The duckling that was pipping from the wrong end died in the shell. I don't know if it could have made it if I had treated it any differently. It had made the least progress of any of those that pipped. I may have rushed it a bit, I had to leave for the afternoon and may have opened it more than it was ready to have done. When I tore/snipped the membrane there was a bit of bleeding. Perhaps if I had been able to give it attention throughout the afternoon and worked with it more gradually it may have survived. Or it's possible that it just wasn't going to survive anyway.
I know that it helped the others to have their original pip holes enlarged just a little bit at first, and then a bit more later. I also spritzed them lightly with water because some of the exposed membranes were getting dry.
I've had hens hatch duck eggs before and never had this problem, so I don't know if it was these particular ducks that have difficulty hatching or if this particular hen wasn't providing the ideal conditions for them to hatch. Maybe she wasn't making it humid enough under her breast.
Last year I had bantams hatching goose eggs. One hen had 2 goslings hatch on their own under her. The other hen, a tiny Dutch Bantam, had a similar issue as this one, where her gosling was pipping & pecking for over a day and not making much progress. But when I enlarged the pip hole just a little the gosling was then able to push herself the rest of the way out.