Let's say my two trays of archers enters a piece of terrain with capacity 2. What happens if Maru adds ranks to them? Do they automatically exit the terrain? Or is Maru prevented from using his ability on them? Maybe they're just crushed to death as they come to life...

Probably falls under the "if the tray cannot be placed it is not added" clause under Maro's rule snip in the box.

The ability just doesn't work. If there isn't room to add a tray, you can't add it.

If you try to expand beyond terrain capacity, there isn't room to add a tray, so it doesn't happen.

I hate to be "that guy," but the rules don't really support this interpretation, even if I think it should probably be ruled that way anyway. The only rule that mentions capacity is 81.1:

"A unit can only enter a piece of terrain in this way if that unit consists of a number of trays equal to or lower than that terrain’s capacity value, and if the terrain is currently unoccupied."

Capacity is defined as a requirement to enter the terrain, not to remain in the terrain. We also know that the physical size of the unit doesn't matter, because in 81.2:

"If the unit ... would not fit entirely inside the physical piece of terrain, the unit is placed off of the play area and a single figure is removed from the unit and placed on the terrain to mark that the unit is occupying it."

This makes it so a 4x1 unit can still enter a forest even though they won't actually fit.

Now, again, I don't think this is how it should work, and I hope an FAQ fixes it to say that a unit can't be increased above the capacity of the terrain. But right now, there is nothing in the rulebook that says a bigger unit can't be in terrain, just that they can't enter.