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No, that's not quite right. Level 5 can be unlocked in the first week, and you can build your game around that, while level 30 is unlocked when it's no longer relevant, and often your hero is much stronger.
On Jebus, it often happens that players go in different directions and explore the two new areas. If you can send a level 30 Isra into a new area with a couple of Necromancer artifacts, you can have maybe 4k skeletons for Fortress Battles compared to 0. It's often possible to avoid Fortress Battles for a very long time.
I believe the Isra/Vido rule would work fine if you were allowed to attack with them but had to dismiss the skeletons. The risk of cheating is very small, in my opinion, and can be revealed if you save every day.
In my opinion, Sir Mulich is not overpowered and could be allowed without any problem. Starting with advanced leadership is not very good, and knights' secondary skills are not that impressive.