There's no set depth to deep litter as it's not the depth of the litter that makes the system work but the nature of it. Depth helps to keep the moisture in the bottom layers, so deeper litter helps preserve the composting action that should be going on, but usually one adds more bedding as needed when it's too moist or when the litter has composted down so far it needs more material to absorb moisture. When using all shavings, it takes a long, long time for composting to occur and ducks have some seriously wet poops, so you might run into trouble with maintaining a really good deep litter unless you vary your bedding materials a little.
Wood ashes won't hurt a bit and can help absorb some moisture.
It helps to avoid flies if you just lightly cover feces each day or every other day in the coop. Just take bedding from a dry place and throw it on a wet place...that's how I do it. No need to stir or disturb the bedding too much to do that...just a light flip of the bedding. DL does better if it's NOT stirred too often, IME.
I'm a believer! We moved our chicks into their coop Easter weekend. We use pine shavings. We turn them over when we think about it. Usually a couple times a week. Throw some new shavings over the top as needed. It really doesn't smell bad. It smells like pine shavings and kind of earthy. Not unpleasant at all. I was raised on a farm. I know what barns that house animals can smell like. I was visiting a friend this weekend and she invited me out to meet her girls. Her coop is large. Its a converted shed. She uses straw and cleans it out copletely once a week. Her birds have more than enough room. But no windows for ventilation. It smelled like a barn. I personally don't mind it since I grew up farming but even my husband noticed how clean ours smells!
I guess what I learned was that neither way is wrong but ours is less work and smells betterThats a WIN in my book!
Yes, that's the big win! No smell, no flies, no hard work, better microbial life, healthier chickens. You might want to suggest to your friend that her chickens will be healthier, as will she, if she opens up that shed to a lot of fresh air....cleaning out all that feces and straw can cause some serious problems for her if the place is not well ventilated. Histoplasmosis is not to be sneezed at, pardon the pun.
