I have also read a lot of pros and cons on what to do about dirty hatching eggs - and I agree with you. You have to do what works best for you. I had a hen sitting on 4 eggs last winter inside the house. Those eggs got so dirty one day towards the end, I wanna say day 17 or 18? that I washed them before leaving my hen and eggs alone for "lockdown". Three of those four eggs hatched, so obviously nothing affected them when I ran them under luke warm water to clean them off. (They were really gross!) LOLThere is a lot of information out there on this matter. But you need to find what works best for you.
From my experience (I've been incubator hatching eggs for over 6 years) do NOT use the excessively dirty eggs. Dirty eggs let off bacteria into the warm incubator. SInce eggs are pourus, they can absorb the bacteria which can result in early embryonic death or ducklings that hatch and then get sick and die.
Some people will use a dry cloth or papertowel and try to get all the loose dirt off the eggs.
Some people wash them completely.
Some people will not touch them at all, simply put them in the incubator the way they are.
I always keep dry, clean bedding where the duck will nest or lay. That way the eggs are usually clean and don't require any washing or wiping.
However, I know that sometimes this is not possible!
For the slightly dirty eggs, I wipe with a dry towel. If I can't get everything off, then I run them under LUKE WARM water and wipe with a wet paper towel. Then I set them at room temp (65-70) to dry.
For me, this has always worked. I get an 80-100% hatch rate with almost every batch I incubate.
I did experiment and did a small batch of 8 eggs that were DIRTY DIRTY.
All developed, as expected.
7 hatched (1 had early death). Of the 7, 1 acted weak for the first few weeks but then got better. The rest were perfectly fine. So you can incubate dirty eggs, but I wouldn't recommend it.