Now I like the idea about Ballista being Drar Summon even when we first suggested it long time ago cuz yea he needs some improvement there but I really think Limited to 5 will be the best Option same for.Gloin and Bifur:-)
I think that the IHs Ballista should be stronger against Single Targets like Trolls and Siege Units and Infantry to be different than Isengard Ballista and unique in its own way! :-) And we shouldn't forget that IRL Ballista was never Anti Building Weapon!
The Chain will provide more interesting and unique Upgrade than the usual Fire Bolt which btw will not to so much DMG against Stone Buildings if we think about it! :-)
I think the idea about Chain Bolts Upgrade providing 2 Ammunition Switch and giving Two Roles ,one.against Single Targets like Trolls and Siege Weapons and the second some Anti Infantry Effect but with CD recharge and Knockback and Area DMG but also those Ballista should be slower than Isengard to represent that they are made from Solid Iron and are much more Heavy which will also Balance them by making them easier Targets! :-)
Also I had this idea which I do not know if it is possible but since it seems that the IHs Ballista was Pulled byt something Chariot/Animal(Ram/s) it might be nice and unique if it is possible to make it so that it be Pulled by 1 may be 2 Rams and still be somewhat Slower because it is much bigger and heavier than Isengard Ballista and when the Ballista should Fire it can have a Function to be immobalized to Fire and after each Shot it will take few seconds before yu can make it star moving again but if you coninue Firing it should be immobalized!
It is just a random idea I got nothing special but if te Team or the Fans like it why not!?
All this things will make the Ballista Balanced and yet Unique than the Isengard one! :-)
This is my opinion on the matter!:-)