I live in a very arid climate, and have never had issues with yew being too dry. Our summers are 12-20% humidity, and my shop is usually 30%. I keep a Hydrometer in my shop to monitor this.
Your trapped just the belly correct? I start all my bows this way, but only for a reference on thickness to keep things even, then continue reducing on the flat section and start rounding the side once I get things even and bending. At 66", and 1 3/4" available width, that's more than plenty for a 26" draw, even 28". Just stick with a flat profile, with radius'd edges