I think some people need a refresher course on Halacha. Many “halachos” that are mentioned are incorrect and can following them can be harmful.
Most Rabbanim say not to sit Shiva today when someone marries out of the religion. It doesn’t have the same implications that it did many years ago. There are many that have returned and or their children became frum becasue of the exposure to frum families.
One is allowed to go into a Mosque, it isn’t a Bais Avodah Zorah. The muslimm religion al pe halacha is a Navi Sheker not Avodah zorah. I would think it dangerous for other reasons for a Yid to go into a Mosque.
One is allowed to go into a known non kosher resturant for various reasons. The problem is if it isn’t known that it is non kosher and someone would see an obviously frum person enter they can think it is Kosher. There also is an issue with eating even kosher food at a non kosher affair because of Seudas Achashvarosh. Sometiems by not going it causes more problems even if it isn’t a monetary one. Each situation needs a Ravs Psak since each situation is different.
Having Goyim/Non frum people at a Simcha is often needed as Darchei Shalom. I personally plan on having a number of these people at my chasunah IYH. They range from coworkers who will be insulted if they aren’t invited and doctors and physical therapists (and staff) without whom I wouldn’t be making any simchos. If they know that I am making a Simcha and they will have to adjust their treatment because I’m now married and they helped me get to where I am and will continue to help me why should I recipricate their devotion to me by inviting them. If this offendse some I hope they won’t come to the wedding because they aren’t there to share in MY simcha becasue my Simcha would be incomplete without these non jewish/frum people there…
Non Jewish/Frum weddings are much more elaborate then Frum ones. They spend so much on the location, decor etc that they have many less people in attendance. If someone complains about a simple (or even not so simple) Bar Mitzvah being so expensive ask them how much they spent on their child’s (or their parents spent on their) sweet sixteen!