If the purpose of this derived version is only to handle individual light on ceilings as explained here, I think you could manage this feature with a plug-in that would modify the property of your choice on room instances. Properties are saved in SH3D files and you may even listen to their change from version 6.4.
A plug-in would also prevent from generating updates for each new version of Sweet Home3D.

If you want to build a portable version, you'll need a macOS computer for some parts as you correctly read (I gave some explanations about building the portable version in a comment of this blog article), but you could avoid it by reusing the existing portable version since you probably just want to change only SweetHome3D.jar file.

Finally, about where to host derived versions, it's up to you to decide. Just keep in mind that "Sweet Home 3D" is a registered trademark owned by eTeks, and it can't be reused anyhow.