The "don't leave this square until I tell you" solution I have described should solve most problems. The idea is that by default a unit has the implicit order to stay in the current square. The player have to give orders to solve some situations.
(Maybe this "hold zone" could be a sub-square in zoom mode, so you would have the hold order this way.)
If I remember correctly, in Starcraft 1, there is the notion of "don't retaliate too far": if you try to lure a unit to your base by attacking it and then running away, after some distance, the enemy unit will move back to its initial spot.
On the other hand, this notion of "don't leave this zone" is part of the simulation, so maybe this zone should be a circle too, instead of a square. Maybe the simulation should be as classic as possible, and eventually some maps (or some user interface) would try to simplify the gameplay, by having some obstacles matching the squares grid if necessary, but this challenge may not be removed by default.
The existing maps made by contributors show varied designs: some maps have very few squares with many obstacles, some are big, open ground, maps. Maybe the design decision to simplify the map or not can be delayed and made available to the map maker.
The new guiding idea would be: the simulation should ignore the user interface; squares are only part of the user interface; so squares shouldn't be part of the simulation at all.