Glad that worked out. Re due dates shifting…there isn’t anything in Asana itself to directly address this, except perhaps if you were to be able to have those be tasks (not subtasks), you had Premium, and used Timeline which has a feature to select multiple tasks and drag them left or right to move the tasks as a unit preserving the relative offsets among them.
I’m not sure if the addon Instagantt might offer more in this area; I believe it at least works well with subtasks.
Other approaches work in some but not other situations. I often just stack tasks in order and don’t worry about assigning due dates at all, if I know I won’t lose track of the project overall. Or just making sure the next task only has a due date; upon its completion, then assign the next one a due date. Multiple task selection can change multiple due dates at once but only to a single value so I don’t think that helps you here.
Future third-party apps may be addressing this further, I think…
Asana2Go & Asana Certified Pro consultant at Trilogi Software