Yeah I get it, the idea of opening shower gifts in front of people makes me really uncomfortable too. I finally realized how much the shower meant to my mom and aunts, so I'm doing it anyway.

I did in a sense "decline" another shower from my FMIL because logistically it would have been very complicated—basically no one else lives in the place where my FMIL wanted to throw it. But what she really wanted was to spend some time with MY friends, so she's throwing a lunch for all of us during the wedding weekend instead, which I'm really looking forward to.

Since what your FMIL really wants is for you to spend time with HER church friends, I think if you decline, you should offer up another time and place for you to meet them. But since a shower at her church is logically the easiest way to do that, it's probably best just to go along with it. Or else think about whether this is really the hill you want to die on.