Nice! You sure catch those bugs quickly!
As for the chance, I think 50% is about right. It's not very often that you encounter a hero with promotions that you would be able to learn, so I think it would work fine.
True, I suppose lowering the bonus would keep those heroes from dominating as much as they are in my game, and would still give them an advantage.
Since we're on the topic of promotions, it would be nice to see some of the more famous heroes start with more promotions. Some, such as Ma Chao, Zhang Fei and Guan Yu have so many promotions that you never get to use some of the unique ones. Those little promotions (Unstoppable, Fighting withdrawal, Heroic ect) are too specific to be a viable choice when bought with experience. However, they seem perfectly balanced as is, so perhaps it would be possible to give these to some heroes for free at start. It's these little perks that make heroes more than buffed up regular units, and makes the game really fun. *cough*Guan Yu*cough* Mr. God o' War ain't doing much in my game at the moment. Isn't he supposed to be charging around the map beheading any hapless generals oppsing him, along with their compatriots, brushing aside gate officers, stroking his beard and haunting those who manage to kill him? I think he deserves to start with Tough, or Assault... he has a great promotion lineup, but lacks uniqueness from the start.