There are plenty of metered parking spots nearby (free after 6PM). Ubers & Lyfts are available if you do not wish to drive.
Yes, there will be a vegetarian dinner option. For a vegan option please let us know ahead of time.
Cocktail attire! Most of the festivities will be outdoors with heaters, there will be a tent for the reception - but be sure to bring a jacket and keep in mind stilettos may not be ideal for walking in the garden.
Hard to say. The average December day in SC is somewhere between 40-65 degrees