You should be able to prune all the way down as low as you want to. As long as there is at least one branch below the prune, or buds. That's pretty much true for tree-like plant, adenium or otherwise. Safest if you still have at least one growing branch below the cut to keep everything active. Even it could be removed later if you add grafts that take or other buds awaken.

Keep the wound dry and add cinnamon or sulfur like you mentioned, or seal it with super glue (which makes a super clean seal). I like to spray with alcohol first and keep everything as sterile as possible, including my knife and my hands. The cut off discards can also be rooted, or some of it used as grafting material on this plant or others.

I'm a big fan of experimenting and not being afraid, nor treating a plant like it's precious if I'm not happy with it or if I have several more like it. So I may be more bold and daring than some here. There's also a ton of reading material online for pruning and training adeniums. Research is your friend. Having something in mind for an end goal of what you want this plant to look like in the coming years is also helpful to guide your training.

That said, if this plant is precious to you and you'd be really upset if you lost it, you can do things in stages, one step at a time so not to shock a plant too much all at once. That's always safest. So I wouldn't prune, wire and transplant all at one go until you really know what you are doing. That's just too much. But I think you are safe to do a hard pruning if it's healthy and you don't do any other big changes at the same time. You should probably back off water a little bit though after cutting as a shocked plant will tend to draw up less, and with fewer leaves, it'll use less water than it was previously.

Thank you. This plant is precious to me, because I grew it from a seed 17 years ago, so I don't really relish losing it. It just never keeps a nice shape, so I thought maybe I could force it to be more compact with closer-spaced limbs. I may do as you suggest and try to do it more slowly, maybe just taking off the top limbs and see what happens.

As far as I'm aware, as long as you have an old leaf scar on a particular portion, buds 'should' grow from that point, so, no leaf scars = no new growths.

Are you thinking of lifting it at all? That will completely shift the aesthetics of this plant, once you see the roots.