I decided to look at the drake problem (that is, that captured drakes are unable to build their respective Chained Drake buildings). I haven't come to a solution, but I think I've learned two things. The first is that the drake that appear in game simply don't have the code to tie them to their buildings - easy enough, and you've probably already found that since the bug report says you're testing the problem. The second thing is that adding the building to the unit's xml doesn't change anything - it still can't build the building. I'm under the impression that this is a problem with the building, because when I changed it to another building that can only be built by captured animals (the elephant stable) it worked fine. Looking at the Civilopedia, all of the Chained Drakes are listed as "can't be built by any civilization" - something that I don't think is listed about any of the other buildings that can only be built by units. I don't know where to find the "can't be built" restriction in the xml, but that might be the second part to the fix. Anyway, hopefully that will help. I may keep experimenting to see if I can figure something out.