I'm an active researcher of the Hittle/Hittle family name.
As I research my name, I find various different places where what looks to have "evolve" into either Hittle or Hittel may well have derived from the German "Huettel."
And, I've also occasionally found places where it looks like other lines than mine may have adopted the Huddell, Huddel, Hudel, Huddle, Huddler, or some other similar variant Englishized version of the name.
Hadler may well be one of those variations. I'm willing to explore any possibilities.
I'm not quite sure what possibilities exist. Actually, I'm sure I don't know what possibilities exist. But if they can be made, I'd love to be able to establish the connection(s).