Most of these weaponry decorations are pretty good, except for the double bladed axe. IRL, double blades were never used because you always hacked your spine in half on the drawback for a big hit.
Uh...yeah...no. Not at all. Double-bitted axes (Bipennis, Labrys) were used all the time, since atleast Minoan times (2000s BC). The Varangian Guard--the elite soldiers of the Byzantine Empire--were famous for them. So, no, there was no risk of chopping your spine in half on the back swing. The axe isn't designed that way, and your arm won't move that far back, unless it's atleast extremely dislocated, if not extremely broken or just torn off.
I said they weren't used often by many people around the world, not that they didn't exist. Double bit axes originated in woddcutting, but wasn't really designed for warfare. Sure, the Byzantine elite guard used them, only because they were elites. Training to use such an unwieldy weapon to be effective would take years to even swing properly.
Then there's the swingback. double bitting an axe is basically adding another axe's weight. To cleave the guy in front of you, without any other injuries to yourself, you need extreme precision.
For my example, imagine the power coming from twisting the torso.
You can try to go safe, and turn just a bit. you'll get about an extra 3 or so degrees from lugging such a huge thing. but once you swing towards the guy, the weight would overpower the hitting force, and probably wouldn't do much outside a small dent in the armor and a bruise. It would tear a huge gash into the guy if he had no armor, but who would rush into battle unarmored if you had intent to kill somebody, rather than being a meat shield?
If you wanted to kill the armor clad guy in front of you, you'd need a bigger swing. Now the kinetic energy from the weight would be greater from the velocity of the swing, and you'd get about 30 more degrees. Your torso isn't supposed to turn that much. You could get your wrist or arm broken, or you could kill the guy next to you and behind you. Now with your new ultra-flexible three jointed arm, your arm would flail towards your back, and with the axe still in your hand, you could cleave your back, like I said before.
Also, if the haft is long enough the blade would end up somewhere to the right of your elbow. Good luck swinging it continuously in a combat situation, though.
A good point, but the fact that the shaft will have to be a fair bit bigger to miss your body, it will just make the swing back harder, due to the low leverage between the weight (axe blade) and your hand.
Protip: You dont swing so far back. Its just stupid because the force goes nowhere. If you do a "Zorn hau" your defence is open as hell and you dont go that far back - 20° maybe 30° under the horizontal line at max but not 90 degree.
edit. A long doublebladed axe is normaly two handed so you wouldnt land in the elbow either if you do such a stupid move.
In the heat of battle, many factors can mess your mind up badly, and you start making bad decisions. You start losing thought of most other things going on around you apart from the things right in front of you.
Away from the double axe talk, but your idea about "Open Defence" is derived from Asian martial arts. I'm talking about massive European army on army combat. Many people were very lightly trained, like how to hold a sword by the right end. That coupled with the confusing heat of battle, and he'd probably not see the fact that your left wide open, or the fact your about to slice him in half.
Well, that should give you some things to think about. And don't just reply that "Double bit axes are dwarfy" or "DF is a fantasy game, therefore the laws of physics don't count".