Question about repotting a Japanese Maple

Well, a nearly 20" container should be fine for your Acer, and the depth looks fine, adding more compost will only give you a couple of inches more, so I would leave it.

I take it, Hstevens, by that you mean gently untangled and spread them out??

Lot's of gardeners have lots of different compost mixtures that they like and use so it is up to your personal experience and like. I use 3 parts General Purpose (Peat Added) compost in my own favourite compost mixture of 3 parts compost, 2 parts sand, and one part grit which gives great drainage while stopping a container filled with only with compost from drying out too quickly...........but it's all a matter of personal choice.
Adding gravel at the bottom is a text book method that will work as good as any other method I guess. Some add small broken pots to the bottom of the container, others use broken polystyrene at the bottom of the containere, and so on.

Just make sure you water the container, keeping the compost damp but not soaked and every now and then give the Acer a weak liquid feed.
Let us know how you get on.