Firstly
So sorry the little guy didn't pull through. I'm just going into lock down on my first batch of duck eggs although I have owned ducks for over 14 years and I am terrified of this all going wrong. I had 12 eggs at the start and I'm down to 3.
I've read Pete's thread that Amiga linked to about a week ago during research for information on how to get it 'right' and I have to say I have found nothing like it on the net, and believe me I've searched. That is the best factsheet for incubating and hatching I have seen anywhere. I have to agree with you when you say that you think the mistake was trying to help. It's of little comfort but being brave enough to share this you will help others in a similar situation and hopefully stop them making this mistake themselves.
There seems to be a lot of threads out there about 'helping' and from my point of view I really want my ducklings to hatch and live but I am coming to the conclusion that more ducks are lost that way than are lost by not helping (Although I accept call ducks are the exception to that). The hatching process takes a long time and we start to panic but actually you never see a mother duck trying to break open the eggs and help out and there are no shortage of ducks in the world. I am going to find it really hard when they start to pip but I've decided that the only point I might help is if the head is clear of the shell and after allowing a decent time if it still can't free the body I might just tweeze the side a very small amount for wiggle room.
As they work at breaking free themselves their lungs start to develop and they get stronger, Pete says the hatching process can last up to 72 hours! It seems that your duckling missed part of that process and it's lungs had not cleared properly or developed enough. That is just me guessing, I don't know for certain. But they have to have time to rest up in between trying to pip and zip.
Shannondee12 made a very important point on one of my threads
~The few that you have going are going to be that much stronger when they hatch. Trust me, it's easier to get over a failed to progress egg than it is to have a weak and sickly duck to die in your hands.
She really has a point, you don't bond with an unhatched egg the same way you do a baby duckling, so it is better to take a step back. My humidity won't go above 55% even with the wash cloths added to the bottom. Not sure if you know this, so forgive me if you do, but humidity isn't the depth of water but the surface area it covers. By adding washcloths you can make that area bigger and that helps the hatch - which you did! Another thing I have noted is that everywhere you go everyone will tell you different things regarding what the humidity should be - but you should take into account that none of us have the exact same factors involved. There is a very good chance that your humidity would have been OK, although believe me I realise and understand your concerns - but until you allow the hatch to run its course without interfering that question won't be answered for the future.
Sadly none of this helps you, you must be devastated and I really am so sorry for you. But please don't give up, get some more eggs and start again. You might not realise it but you really have learnt a lot about incubating and hatching during this and that will put you in good stead for the future. We all make mistakes and you did this with the best possible intentions in the world, no one can fault you for that and you mustn't blame yourself in any way.
One other quote from Shannondee12 that I really have taken to heart ~
My very first duckling hatch was with 6 shipped eggs. I didn't know anything about hatching ducks or to let the eggs settle for 24 hours to let the air cells re-attach. I had the old stand by stryofoam bator that was still air, no fan. I didn't monitor humidity and even let the water run dry in the bottom by accident. The power went out a few times dropping the temps in the bator to under 70. I did everything wrong that I could possible do wrong besides drop the eggs on the floor. I still had 2 out of six hatch. The moral of the story, don't over think it.
Good luck for the future Krissy