I totally understand your struggles, I have a few LDS friends and they find sleeves challenging, most may not understand that you need at least mid bicep and it can't be sheer/lace etc or fluttering.

I think floral maxi dress would be appropriate unless the fabric is more tshirt like, so maybe not dressy enough. It looks like the sleeves will be long enough though. I think the Old Navy dress is too casual for a wedding.