Hi Frank!
Sorry that you’ve had to wait so long for reply. With gbXML there are issues but not where you are looking. There is a native way of exporting gbXML from ArchiCAD, and it exist since ArchiCAD 20. Just go to Energy Evaluation under the Design tab. Next to "Start Energy Simulation there’s a little arrow. After you’ve done with AC simulation you can easly export gbXML. Problem is that this file is worth absolutlety nothing - at least in my case.
Yes, with internal walls assuming that all of them are the same is definietly fastest way and I’d say still pretty accurate.
With Daylight simulation it seems that you know what’s going on so I’ll leave you at that. 
Good luck with your project! And come back if you’ll have any questions - I’ll whatever I can. And of course tell us when you’ll find something on yourself! For now that’s the only way for us to learn those tools.