I don’t think I would bother with folding spokes for space saving, the spokes are shorter than the frame or the rim sections if split in three.
For joining two spokes into a longer spoke I’ve cut a section to add to a short spoke and rolled new tread then joined the two sections with a brass nipple. You can see an example in my 32" wheel project. It worked OK and everything is strait so you aren’t adding any new points of flex into the system. I wasn’t the first to do this, but I don’t recall who else was doing it.
The rope spokes are cool but I don’t think I would want to deal with them settling each time on repeated builds. Perhaps a great option for odd wheel sizes that won’t be rebuilt often though!