With the Grand Magus, you can develop anything. In the Campaigns, I even developed Empathy for Zehir. It's true, I spent a huge amount of money before I got Leadership. I easily developed the Special Ability (SA) for Gotay (with the help of the Grand Magus). In the vanilla version, I tried to develop the SA for Agraila three times. It worked once. And in version 3.0, I tried once, and it worked right away. I didn't even try to develop the SA for the others in versions 1.3-1.5. Either I didn't like the SA itself, or the path to it, or both.
I agree that my chances are decreasing. But I was very afraid to start the 3rd mission without Magic Affinity. In the first 2 weeks, the hero needs a lot of mana. In any case, I will develop what leads to the SA. In 3.0, there are almost no useless abilities. It's a pity that I failed with both witches in the 2nd mission. I don't remember how many more will be on Raileigh's path until the end of the 6th Campaign. And there is also a high probability that the path to the SA will fail much earlier. I didn't want to develop the SA for Isabel, Godric, and Marcel even in 3.0. I won't do it for Zehir either.
In my opinion, you should only develop the SA when there are only excellent abilities on the way to it. Otherwise, you can end up with a hero who is not very well developed, and you still won't get the SA. Everything depends on luck.