This comment is very late, but maybe you'll see it in your threads. They discovered the pool of heroes didn't take exponential amounts of growth to increase, despite the pick possibilities moving up exponentially. That was one of the results of their blog - training each new hero was a linear increase in difficulty. They stopped at 18 because that's how many they had trained when the competition start date hit.

For the limitations, divine raiper and bottle are items with unique interactions. Divine rapier drops on death, and bottle has unique interactions with elements on the map that require interacting with the environment in a specific way.

Illusions are the same as LeBlanc's Mirror Image skill. Just copies of the hero that either deal no or limited damage, but can't cast spells. Likely they'd have to train the model to include a evaluator on the likelihood of a unit being an illusion rather than a hero.

Couriers bring items from the shop in base to your hero, so you don't have to return to shop. They are killable however, and if they are holding items when they are killed those items will be inaccessible for 3 minutes. They made them invulnerable because the bots would re-buy items that were inaccessible. Invulnerable couriers can be exploited however, thus the rule.

Scan is a global ward that only tells you whether or not someone is in the area (doesn't give you vision), and lasts 5 seconds.