Are description keys used for point features and figure styles used for linear features? Do you need a description key for each figure style?
Where does one control whether a point is used in the TIN? I assume the figure style database manger controls whether a line is used in the dtm of just for topo by checking or unchecking the Use As Breakline toggle.
Description keys are critical if you want to place symbols on points and have those symbols scale or rotate based on an additional parameter in the raw description. Good for trees where you want to display the symbol to represent the trunk or dripline.
If you just need a symbol, but it does not need to change sizes for different points, you can use Point Style to apply the symbol and have your Point Group reference that style.
If you like to separate points to layers, Description Keys will do this. I prefer to just use Point Groups and control their visibility by Point Group Display Heirarchy, but that's just me. Others like to use layers.
To determine whether a point is used in a TIN, create a point group of all the points you want included in the TIN and add that point group to the surface.
The Breakline option of the Figure Prefix database just tells it this is a 3D object, appropriate for use as a breakline. You need to add them to your surface the same as you would if they were 3D polylines.