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I tested the situation with Jelu. I gave him the following artifacts: 4 elemental spheres, 4 elemental books, a vulnerability medal, a wizard's well, a potion of life, and shackles. I couldn't manage to remove the sphere of prohibition. Interestingly, in the only empty slot next to the sniper bow and crossbow, he adds the shackles. Also, I couldn't achieve the bugged troop merging, although I read that it can be done somehow. After capturing a mage's or wood elf's stronghold, he merges the snipers into 1 stack – 69993 units.
In general, Jelu is set to a radius and cannot reach any castle, so you can defeat him by taking all the cities, but then you won't get bonus angels in the huts.
The author also writes at the beginning that you should free Caitlin, as she can teach magic. But she has learning instead of literacy :D