It sounds perfect. I agree, someone just put "bridesmaid dress" in there to draw internet searches. A dress is a dress, for that matter even if the wearer originally got it as a bridesmaid dress. You are avoiding looking like the bridesmaids at this wedding, and that's all that matters.
The white shawl will be just fine, too. Don't give that a second thought. A white shawl simply doesn't make you look like you're competing with the bride.
Let's see a picture! (Not in order to make sure it's fine -- it is -- it's just fun!)