I have a yew bow that has the blue steaks in one limb. I don't think that is your problem, the violations are. If you thin the sapwood evenly you might reduce those violations and either not need to back or use the rawhide for insurance. If you are going to continue to worry about it all then you'd be wise to remove the sapwood and sinew back as already mentioned. If there's not enough heartwood for sinew backing then there's probably not enough for sapwood backing.