You're gonna need to reduce that sapwood by a HUGE amount, as it's massively thick at this stage. I'm sure you've read/heard often that yew can take a lot of violations unlike some other woods, so you've basically got two options:
Keep the slope as it is by chasing rings and deal with the twist you'll get as you go. It shouldn't be too catastrophic but some heat twisting might be required
Or
Jump through the sapwood ignoring rings completely so you end up with a flat back. That's what I would do. As long as the violations run the length of the bow you won't have any problems as the shear forces on the layers of wood won't be across the bow but along it. It's violations running perpendicular to the profile you want to worry about.
You could try a few other methods if this is a spare piece of wood, such as twisting the entire bow within the design and chopping edges away so that you essentially force the bow to sit flat within a twisted stave.
As a word of advice when dealing with a piece of wood like this, following "reflex" isn't always the only option. That's a HUGE amount of reflex, and will almost certainly cause a lot of tillering problems if you're not used to it. You'll think the bow is far heavier than it is, as the reflex will ramp up the early draw-weight, making you take more wood off than you think, and you'll suddenly find yourself well under-weight by the end. If you want this to be a kid's bow you've got a hell of a fight on your hands. It's hard enough making a low-poundage bow with a straight stave, let alone factoring in massive reflex.
What I would have done with the piece of yew you photographed in your first post is flip the log over and use the "deflex" side as the back. The rings are tighter, which means less work on reducing the sapwood plus more power as essentially it's got a higher rpi count than the other side. I would have flipped the tips back on themselves and ended up with a stunning little reflex/deflex shortbow which you could have used yourself. Don't always get drawn in by this magical "reflex" term - it's not always a good thing!
