I mix it all at once, mainly because I make my mix from scratch. It's best mixed all the way through, but doing the top few inches will work also. Just takes a bit longer for the lime to work its way down in the mix.
I use both the powdered and the pellets. The powdered mainly for the mixes. I measure out the peat, then add the powdered lime, mix well and then go on adding all the other ingredients. The pellets are mainly for the soil garden and if any top dressing is needed for long term plants like tomatoes. Much easier than trying to spread powder by hand, especially if there's a breeze.
My garden soil is acidic, but nothing like the peat based mix I use. On the soil, I apply about 1/2 as much, 1/2 as often as the mix.
Wet