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!
Indeed, if you look at the folding penny farthing you will see that none of the spokes are removed at all, or retensioned when the wheel is put back together. This is the time saver
My idea about folding spoke had only a goal to make assembling process so easy as possible. Short part of spokes always left on the hub, long part easy removes and installs back. For me, it is not pleasant work to input common spoke into a hub holes and to bend it to provide it between another spokes. My construction lets to do it very easy. I don’t need now to bend spoke every time, and I can easy install or remove any of spokes in any time. Of course here no differense in demensions for storage.
The way with extra thread of couse is reliable, but it is not available for home make, if you have not thread roller machine.
I’ve used Sapim Double Square nipples on the last few wheels I’ve built (‘conventional’ use not for joining spokes!) – they would probably give a bit more thread engagement for this type of application, I think they only come in aluminium though.