SAPWOOD for sure. And Siberian elm isn't bad wood at all.
Having done a lot of elm sapling bows, your current issue is that you are now working with the lowest common denominator, know what I mean? Somewhere that belly split to a narrow lengthwise ridge and once you get it down, there's gonna be less wood than you would like, wish, or a place where it's already narrowest, and that is now you default maximum width, etc.... That's OK. You just want to get the most out of this stave.
So, you gotta go for it. Find the narrowest, thinnest, and longest dimensions you ACTUALLY have, and trim up the stave to correspond to those. Take all the width you can get for now. Square it up to the back as best you can, find the peak of the crown and mark your centerline. Next work the thickness until the whole thing is consistent, and go for the BITH bow.
One approach I like is that once the stave is same width and same thickness all along, you KNOW it's going to bend in the middle, right? So, now, methodically thin the entire stave until the middle starts to flex. Just barely flex. I do something like mentally or with crayon mark out the bow in five equal sections, each 20% of the length. Once that middle 20% flexes AT ALL, get that whole middle section bending the same and then mark or tape it off, and don't touch it. Move to the next section on each side of the middle. Methodically remove the same amount of wood from both limbs along the whole bow (except the middle, of course). Do this by either floor tillering or by stepping on a longstring. BARELY flexing....
I use the method where you rasp until rasp marks cover the whole belly, then scrape off the rasp marks until none are visible. Same each side. Eventually, those sections will flex, Just a little like the middle did. Usually one will and the other won't. Leave the one that flaxes alone and methodically remove wood from the stiff one. WHOLE LIMB, not just the section. Say after two passes, or five, it'll flex, too. Stop here and narrow the bow down side to side in that last section. Check again. At this stage it's usually ll bending some, but you are only flexing the tips a few inches.
If the stave feels ridiculously heavy, make several passes of rasping and scraping the whole bow. Take it all off slowly and consistently. From here work it to brace height, making sure yu aren't pulling it much above intended draw weight. Once braced, the whole bow will be bending, but not perfectly, and it's easy to see where it needs removal. Tiller it out on tree or stick as normal, pulling it only to the intended draw weight. That's how I get a rough stave to the tiller stick, and it works well if you have never done a split stave before.
