What they used to call informal, but what most of us would call a good suit. No black tie, please, unless you are a write-in candidate for groom.
Open, of course.
Yes: one-way! We're moving across the country a few weeks after this.
April 30 Update: Yes, we will be inside in this weather! There's a perfectly lovely room in the building for our ceremony. Please, come on in and come downstairs. Same time, same address, just out of this wind and rain!