I do think that if you select say next unit in the unit scroller then updating the selected territory information along the bottom could be a good idea
That actually already happens. If you use the hotkeys, the territory bottom bar status stays in sync with the scroller. But not the other way round though, if you use mouse-hover the scroller becomes "out of date". Having the scroller act like the territor tab I'm starting to think would be a much more conistent UX. The reason unit counts were omitted is because of how the scroller was rendered, it was generally known that more than one or two units and truncating after three would mean you could not clearly see the units. With the upcoming layout change you can see them pretty clearly, meaning a unit count would not be as misleading and potentially really useful now.
The biggest challenge is just space. As a feature like this is most useful for maps that have too many units in territories on the map to easily see.
True, but we should not also forget the unloading onto islands case. That can is quite common where units still on transports look like they've offloaded.
Which means you are probably talking 10-20 unit types
Experimenting quite a bit with unit scroller, I've found the typical count is 1-3, sometimes 4. More than 4 is not common. A worst case on NWO, perhaps a map with the most unique unit counts does get you up to the 8~10 range. That scenario was a bit contrived, though could happen, but is also not a common one.
I suspect we probably can create a cut-off for how many units are displayed and put some ellipses or a place a label (20 more units..) that has some hovertext to show them all. There is quite a bit of extra space not being used if we are talking about miniaturized unit icons.
Re: battle calc
The battle calc for this purpose I think is a symptom of a problem. For new players, they're not going to know to use it or even the hotkey for it yet. For veteran's, falling back to it is a bad sign. In part the idea is to think of ways to fix this. (I'd agree that map fixes are good way to go, but are their own dedicated ideas, not all maps will be perfect)