Are attacking hyper skills mainly designed for bossing only?

Keydown means just that, you have to hold the key down for it...
You'd have to hold the key down on UD to move while using it, just as you'd hold it down to use it while standing in one spot, like other hurricane skills.

Saying it's not a keydown implies you press the key once to activate it, and again to stop. Also, we have to stop using UD in order to FJ, unless BD and asura are different that much (I know one can jump during those two) we can assume FJ during is impossible.

You don't hold the key down for asura. You press the key to start it and it lasts as long as the buff icon lasts, then stops, without pressing again, like a buff does.