About the First Vertex issue:
I have a setup where I use a Selector to switch the Corner Segments based on vertex type (I use the Index parameter and get the value by the expression: XVertexType).
I noticed that the First Vertex returns the wrong value if the next spline segment is NOT set to MatID=1.
I don't have time to investigate more but I'll see for weekdays to make some simple examples.
It is not critical but it took me some time to see it through.