the garrison build definitely makes sense for lich players building spiders or even worse rakans buiding turtles. to enhance the idea a bit: as soon as you do build a garrison unit, this one should show up on the wall (like injured units, just with a flag "queued" or "building", to prevent over-capping the wall.
the plundering is a different issue, mostly since the queues for units and buildings / research are different. units consume all queued res at once, while buildings / research only consumes the res for the current project. this should get unified.