1) Do +x Engagement or +x Shooting stack? This would make a big difference with units comprised of only Persian Immortals or Persian Archers. (Would a unit of three Persian archers roll two dice or four dice when shooting?)
I play that way.
3)With the Tough trait who chooses the dice to be re-rolled? You or your opponent? This could make the difference between a single wound or an entire unit being destroyed.
If your opponent chooses the dice then the trait would be pretty useless so I play that you choose.