Big One – what does the phrase “normally assur” mean? Does it mean that you havn’t done it – or your community? Does that mean that anything your community hasn’t done is prohibited until proved otherwise? Cute. But not Halacha. Respect the Halacha. Halacha only respects reason. Not emotions.
1- Entering a conservative temple; can anybody make a reasonable argument that such a thing would be prohibited? Remember, we need an actually issur, d’orayasah or d’rabban. Not ” mamish avadeh”. My Shulchan Aruch is missing that chelek. Beis Minus? Maybe. Avraham Yehoshua Heschel was certainly an apikporus, as were other conservative leaders. But they are not worshiping another god. They deny Torah principles, which may be Kefirah – but is not worse then a university – certainly one could enter. I believe it is permitted, though should only be done when there are no other options.
2- Attending an intermarriage – I could hear it being forbidden; you are joining a celebration, and by definition celebrating, a horrible issur (even more then an issur, an opting out of khal yisrael).
3-An event which is not tzinius. Yes, we all must avoid nisyonos shemiras einayim. But to attend an event which has immodest people there is categorically permissible, if the event is unique ( hopefully your brother will not have a second marriage), under the principle of Lekeh Darka Acharina, and then Shemiras Einyanim is obligatory. Business people rely on this everyday.
And if a not-yet-frum relative is marrying Jewish, it should be strongly celebrated and encouraged.
And hopefully, if you are clever, you can invalidate the marriage halachiclly (get two brothers to witnesses, whatever) and save Khal Yisrael some mamzerim if things don’t work out. Offer to officiate.
4- Eating in a non-kosher home – with mutual respect, disposable goods, saran wrap & silver foil, you can easily prepare a kosher meal in a treif kitchen. Ever eat airplane food?
5-Insisting on tzinius – would you pasul a FFB with non-frum relatives?
PS: Two ring kiddushin is a machlokes haposkim.
Any other great, horrible, issurim? After all, us FFBs have a flawless marriage record. I think BTs have added a lot to khal yisrael, besides for the tremendous kiddush hashem