Particular thanks for the tip about bypassing the needle bar thread guide with one thread when using a twin needle. I've just tried it on a scrap, and it worked nicely. It's much easier than what I might otherwise have tried: buying some S-twist thread for one needle while keeping Z-twist thread in the other needle.
I’ve never seen that elastic thread. Thank you for showing this. I have seen a thin elastic cord which I zigzag across, but it couldn’t be used in a bobbin.
When using a twin needle, I always have to use some kind of wash away stabilizer, and you have to make sure to catch the hem fabric on the back with both needles or the fabric will tunnel on the top.