If they can force you to login to your laptop, they usually don't have to force you to login to your Dropbox account: Most users are already logged in to Dropbox and all Dropbox data is available in the Dropbox folder by default anyway.
Yes that's true. I wonder about the case where the data is not present on the device. I've seen lots of people assert that the government would be powerless in that case. I think this assertion is often made on the basis that the alternative is too horrible to imagine. I hope there are more compelling reasons why this would not be legal but I am doubtful.
For example, I have heard of border agents taking phones and calling contacts on them. This is a way of getting data about you from another location. From there it seems like a small step for border agents having the right to get access to your data at the border if it is available to you when you are crossing the border regardless of its physical location.
