I haven't found 2d-fillet in the API and suspect that it doesn't exist. 1. Am I right? 2. If so, has anyone written a script to create a 2d fillet?

Not available right now. You would need to do the math that Alibre does internally and draw it from the pieces yourself. Are you wanting to 2d fillet a horizontal and a vertical line? (Easy) Or two lines at arbitrary angles? (Hard) Or anything other than two lines? (Extremely Difficult)

Nope. It seems sketch constraints added with AlibreScript do not get resolved like those added by the user. They get created but don't move the geometry into the correct location for the constraint to exist.

I need it for two connected lines at arbitrary angles. My approach to the math is to envision two lines parallel to the original and at a distance equal to the radius of the fillet. Then find the point of intersection of the two new lines, which will be the centre of the fillet arc. Next, envision perpendiculars to the original lines which will provide the end points of the arc. My hope is to simplify the math to provide formulas for the three essential points in terms of the end points of the original lines, and then have my script use the formulas directly in drawing the arc and trimming the original lines. I guess it's more of an analytic geometry question than a programming question. I'm making progress but my math is rusty. If anyone is adept at this sort of thing I'd appreciate their input.