I perused the coreunits.xml and I agree with your assessment about the spawn rating. I looked at several of my games and it appears that the spawn rating is as follows
spawn rating 1 : level 1 hero
spawn rating 2 : level 3 hero
spawn rating 3 : level 5 hero
spawn rating 4 : level 7 hero
spawn rating 5 : level 9 hero
I looked at each of my maps (cheating of course) and could not find anybody with the hasessence tag. I wonder if you change this to spawnrating # if they will show up on the map.