When you're not sure, it's all about dressing to blend in either direction.
My suggestion would be a jean or khaki skirt with a twinset type top where you can wear just the shell if everyone else is majorly casual or you can wear it as the set if everyone is more formal. If you can bring small tote bags with you (I wouldn't bring a full out weekend bag or anything...) I would suggest also bringing both a fun, casual necklace (maybe something big and chunky) and some pearls or a gold chain, and also a casual pair of shoes (nice leather, NOT PLASTIC flip-flops/sandals [if it's warn enough]) and a pair of ballet flats or something similar.
Either way, if they're casual, you're still in denim/khaki, plain shirt, funkyish necklace and casual shoes...and if they're formal, you're in a skirt, twinset, pearls, and nice shoes. Lots of choices with minimal toting extra stuff around.
ETA: I was still typing when you were posting...what you picked out sounds good, except I think if you've got a good pair of more feminine, ALREADY BROKEN IN shoes, I think that might be better. But, please do not wear new shoes. You'll hate yourself the next day.
Something like this, maybe:
or:
