Dependency lines don't have a built-in functionality on hover, although I've prepared an ad hoc solution on how you can display a delete button and remove hovered dependency line: https://jsfiddle.net/BlackLabel/os3rLa4q/
As for modyfying lines length by dragging them like bar points, there is no easy way of achieving it and it goes beyond scope of support on our forum. If you need help with your implementation, you can try to look for it on the more general forums like StackOverflow or contact the Black Label company that specializes in Highcharts custom projects. See official Black Label site: https://blacklabel.pl
In case of any other questions related to Highcharts functionality feel free to contact us anytime.