A full research into this last night and we determined this is an alternative method of creating the 'has already researched tech' button display. I guess this was added at some point in case we wanted to have a different approach to this button display entirely (such as something like greyscaling the image or something.) I want to enable this in the tech tree main button display on religious techs. It may never need be something different than the main <button> in Religions but setting it up for use, if we ever wanted it to differ it could be. If that makes sense. At the moment, the references here will be updated to the same as the main <button> in Religions which will also be the same as the button in tech infos.
The big difference I'm asking for here is for the religioninfos.xml <techbutton> tag reference be used as the tech button on the tech tree tech display when the religion isn't researched. And if it is, then pull on GenericTechButton instead. Right now, it does NOT do this swap out with the current tech tree... it's asking all techs, including religious ones, to refer directly to the techinfo <button> definition. I've found the python on this and ran tests to confirm. Perhaps an old tech tree we had did it right but it's not correct at the moment.
As for the subjects you brought up, you're the master in this dept. I'm good with what you want to do! Your reasoning makes sense.