Thorsten I have been looking at the Spirit of Thuringia by Shepelev & the Schiffer book by Dabrowski. I will have to dig around some more in those as the stories are interesting. I liked the way one Horten brother was manipulating funding with fictitious units
I also tried to find something on youtube, but there wasn't much. Some good Luft46 game sims of it tho.
Horten Ho-2 Flying Wing Test Flight 1935 - YouTube
Ho 229 Us army Fly test - YouTube
Hitlers Stealth fighter - Horten 229 - YouTube
My main question on the real deal was yaw, roll must 'replace' some of the lateral motion created by a rudder.... I can see that could have a big impact in dog-fights, but I guess a resident aeronautics expert will be better equipped to explain why the design is less common.
I hope your project does get airborne, I may even be available to beta build something if you need volunteers.