Nice substantitive argument...
Regarding the clause, “Congress shall pass no law respecting an establishment of religion...”
I would argue the following:
1. If indeed the intent of the Constitution was Federal Gov’t religious neutrality, doesn’t it follow that State gov’ts WITHIN the Federation should be subject to the same interpretation?
2. A nativity scene in a State building is essentially “respecting an establishment of one particular religion.” Consider that there are Jews, Hindus, Buddhists, etc. who are State residents and taxpayers and they may object to the sanctification of the Nativity Scene by the State.
BTW, I welcome counter-arguments/discussions such as yours having substance...