Is there a quick way to select multiple tasks, and edit their labels/priority all at once (just like how you can move them all to a project)?
As you already know, you can CTRL click tasks and they will provide a new menu to edit those tasks. We will consider adding more options from within that menu in future updates.
Is there a way of editing multiple tasks on iphone?
Unfortunately, there's no way to edit multiple tasks as now in the iOS App. You can however add feature request in the votebox here: https://todoist.com/Vote/. In here, other fellow Todoist users can vote for ideas that they like.