As regards the varying heights of models, since the spacing for revision tiles below the original one is tightly controlled (3), I assume that any very tall tiles would be single-occurence tiles (no way to place revision tiles below if too tall?)?
no. It doesn't care as long as it is on some multiple of your tile size. You can skip multiples and it shouldn't matter. If you find that it breaks something, let me know and I will fix it.
And Tiled could care less if the tiles in question were much above the "ground" layer?
Thats right, tiled doesn't care if the models (or even the tile images) actually integrate. It is just doing what it is told as defined in the terrain sets in tiled.
For the tileset you made in Maya, to get the different color layers did you extrude out the curve for each layer and then manipulated it to color it, or how was that accomplished? Also, the curves were done manipulating/adding vertices?
I don't usually use 'curves', since they tend to create needlessly complex geometry. I cut each vertex in the tiles by hand using the split polygon tool.
And how did you assign the color(s) to only that specific area?
That is the art of UV mapping geometry to a 2D texture. Window > UV Map Editor.
This is well outside the scope of Tiled to Unity though. Sounds like you have a long road ahead
But there should be many tutorials online to help you.