Just a few opinions from someone who goes outside the wire frequently, While the axe itself is a pretty nice design, I think the sheath might be over engineered. I wouldn't worry so much about the dual retention issue, as the last combat airborne op was in WWII. As long as the axe will stay put fairly well and the sharp points are covered you should be alright. You have to remember that space is at a premium when it comes to personal gear. If you make your sheath too big and cumbersome, it won't matter how great of a tool you've made because no one will have room to carry it. I would keep the sheath simple in a way which allows me to attach it to molle gear. regularly spaced holes and slots make this easy using 550 cord, but if you really want to be highspeed, find out a way to attach Molle attachments to the back. Also, a retention strap in the back is ok. I personally wouldn't worry too much about covering the prybar, as it shouldn't be super sharp anyway. If you're looking for a place to put the wirecutter notch, I think a good spot for it would be to set it up as a split in the prybar. This way you could also use it for pulling nails or something along those lines (although I'm not really sure when I'd need to pull nails in combat) As for the saw, it's nice and all, but I'm not sure how I would get decent leverage on it without cutting my hands. Oh, I also think the G10 scales are a MUCH better idea than cord wrap. I think it's important to note that an Axe is more likely to be used as a tool than a weapon outside the wire, so a steel which is strong enough to, say, cut open a vehicle or something along those lines would be best. Just my two cents as someone who spends a decent amount of time on the ground.
Oh, by the way, what CAD program do you use?