Hi friend, thanks for posting the extrude taper problem file.

So yes that is caused by your polyline being fragmented into multiple segments. If you use Edit > Separate on it you should see that some of the areas that look like lines are actually 2 individual collinear line segments instead of just one single line:



I've attached a .3dm file where I've fixed those up to be single lines and if you do a tapered extrude on this one you shouldn't see that problem anymore.

I'll see about your fillet questions a little later on.

- Michael