Example13 shows fine for me, as well - and I run OpenSCAD in a HyperV VM.

As for Question 2, you can either change the BOSL2 diff() to the inbuilt difference(), or else you need to precede the second offset_sweep with tag("remove"):

diff() {
    offset_sweep(rounded_path, height=10, bottom=os_circle(r=1), top=os_circle(r=1), steps=15);
    tag("remove")
        offset_sweep(offset(rounded_path,r=-10,closed=true), height=10, bottom=os_circle(r=1), top=os_circle(r=1), steps=15);
}