I'm not sure how to do auto-summon but I like the idea.
As for the target casting the idea there is to be able to "construct" a defensive line when your base is under attack. Because the cast range of each structure is the same as it's blight spread radius you have a large area to cast in. So if you have a critical structure under attack, say a Conduit, you can summon 4 Shamblers in a line between the enemy units and the Conduit.