Adding the fruits in increments like that would be ok but probably unnecesary, you might want to consider upping the quantity to 10+ pounds and put it all or at least most in the secondary with the vanilla, that seems to really maximize the amount of flavor and aroma you get, backsweetening with a little of the juice at the end like you described will boost the flavor as well, you will probably have to let it clear again to prevent haze.
Going with water over apple juice is good to help keep you in the melomel category, the juice sort of pushes it into a cyser, which is great too but if you want a melomel then water is the way to go.
The campden tabs or kmeta in the begining is fine, not really necessary as its also an antioxidant and in the begining you will want as much oxygen in there as possible, even adding more with initial daily degassing/aeration.
Step feeding is always a plus for your nutrients, can be done at the same time as aeration.
I personally like tannins in melomels, and a lot of meads, I often use oak as a source but if I am not I also use a powdered source, So I definetely like that idea.
here are a couple links to compare to 71B-1112 and K1-V1116