Hi KrisB,
There are a lot of ways to approach nutrients. DAP and Fermaid K work, so do organic nutrients like Fermaid O, but you can get proper yeast nitrogen by adding pollen, or fruit, or malt and there are many ways to address it. Part of the reason many recipes list using DAP plus Fermaid K is that in commercial winemaking the TTB limits the amount of thiamine addition to 0.6 mg/L. Providing all the nutrient with Fermaid K would likely send you past that limit so people would use a combination of DAP and Fermaid K so as not to exceed that limit. People just adopted those practices into meadcrafting.
Over time, I have found that I like my results more with less DAP and more organic nutrient. I started testing using Fermaid O alone before TOSNA protocols had been described. In cases where I am not pushing the yeast available nitrogen (YAN) beyond 150-200 ppm, I may continue to use only organic nitrogen. When I go higher, I will typically use Fermaid K because the DAP in the Fermaid K allows you to get the nitrogen up much higher with smaller amounts added. I don't care about TTB limits on Thiamine. I am not the least bit concerned about getting too much thiamine as it is water soluble and immediately excreted through the kidneys. I haven't added DAP (other than in Fermaid K) in years.
Key points to know are:
DAP 1 g/L gives 200 ppm YAN
Fermaid K 1 g/L gives 100 ppm YAN
Fermaid O 1 g/L gives 50 ppm YAN
With that you can mix and match to get the level you want.
Most fermentations need 150-250 ppm YAN, depending on the yeast strain and strength of must, but if you really want to push high ABV yeast, you may need quite a bit more.
I hope that helps to clarify some of the confusion.
Medsen