I'm not convinced they will return to tiers, partly because of the reasons scottishskifun said, but also because of the interconnected nature of so much of Scotland. Wherever you draw the border, there will be people who live/ work/ shop across it and that's so difficult to control. So even if one area has low "numbers" there will be people from higher areas with a valid reason to be moving into and around it. Plus it's human nature to go to an area with shops and restaurants if there aren't any open in your area! The possible exception could be the islands I suppose, but even then there are valid reasons for travel to and from the mainland (like the vaccine crews!) I don't know what the solution is, though - but schools need to go back everywhere frankly before they start thinking about who can have pubs back
I think settling on a nationwide order to reopen things in, and then doing it everywhere at the same time is probably what they will do, though.
But yes, as you say, all council areas are now in at least level 3, and many in level 2 just looking at the numbers. So why schools and nurseries are still closed is a mystery to me.