It is kind of the case depending on how you look at it :  We could say that units aren't actually pushed, they just look at every neighbor that look too close to them, then they move away from them. Consuming all forces, they calculated tells them where they have to go for this turn. So it's them actually moving away.. Well since all that is a bit some abstraction of physical events probably there are as many way to phrase it as you want.  I'm glad for the involvement, it shows that this is an interesting topic and that many things will sprout out of it.  In fact, a big highlight of this project is that it will be easy, even from a mod, to plug in forces into UnitMotion. Examples : weather, spells (attractions, repulsion)...

ok, another random guess: Maybe we could use an additional force, where an idle unit clears the path for an allied moving unit, before they are colliding, when it's blocking the way? The blocking unit would just slighty move to the side, but only if there is space available

I see, tangential forces would need friction to act. And I guess including that might cause a bunch of other problems, like units getting quite stuck.

Like a simplification that doesn't account for friction? 

There was a guy named Enkeli who knew how to do that for AOE II.   https://www.indiegogo.com/es/projects/enkelikuoleman/rise-of-kings-medieval-warfare He knows how to convert 3D models to Sprite models.