I just got this:
GetImage error: Request for frame not present in CAnimation! File name: COURTYARD/BATTLE/WMCAMERA, Group: 13, Frame: 0
After that the game freezed.
Maybe with latest builds unknow error occuded.
Reproducable? Or just one-time issue?
One-time issue. At least not that easily reproducible. It was a fight against the camera obscura specialist, and the missing frame was something around this camera being destroyed. According to the message it is a missing bitmap in the .def file.
I have never played that hero myself. Although the idea is nice, it is completely lacking any support from the town. So i did never choose that hero.
Edit: A suggestion for the author of courtyard: Move the alchemy cart to the alchemy shop, which currently has no function, and make the camera obscura byable at the blacksmith (instead of the alchemy cart). Just a suggestion though 
A question about new skills:
There is the field “gainChance” with the subfields “might” and “magic”. Is it possible to add another gain chance for a specific faction or even a specific hero? The wiki has no info about that 
Impossible to do. VCMI doesn’t allow to buy custom war machine in special building.
Unfortunetelly no
Hmmm, the Stronghold has this feature. You can buy ballista and ammo cart (in the upgrade).
Ballista Yard type special building is not configurable. You can buy only ballista as second war machine.