If your invite says "and Guest" then yes. Accommodations will only be made for those explicitly listed on the invitation.
Dress code will be semi-formal, wear your favorite dress (long or short) or suit
The venue has plenty of parking spaces available with extra spaces for overflow
Yes, just refill the form with your name and what your status is changing to.
If a member of your party is unable to attend, please refrain from bringing guests outside of your listed party.