At risk of getting sucked into this increasing toxic cesspool, I’d like to address one narrow point…
Avi K:
Would you sell a pen to a non-observant Jew?
There is a factor that you have not addressed that I believe greatly impacts this question. If an irreligious Jew (or a religious one, for that matter) has no other way of doing an action that is contrary to halacha unless you help him, you are not allowed to enable him to do so. That would be mesaya li’yeday aveira. However, if the said person will be able to violate the halacha even if you do not help him, there is no issur of lifnei iver lo siten michshol.