Have your own transportation, so you can leave if things get too uncomfortable.
Yes, you can do that. You can leave before they cut the cake. Your sobriety is the most important thing. Everyone else will be focused on the happy couple, or on their own good time. If you leave early, my bet is that no one would even notice.
Bring along some phone numbers of sober friends. Just in case you need reinforcement.
Practice saying, "Just plain coke, please," or "Just plain soda water with a twist." Practice saying, "No, thanks, I'm good"--while holding up your glass of soda when it's time for the toast or if you are offered champagne.
If people ask why you aren't drinking, have an answer ready. My usual response is, "I decided it isn't good for me." Period. No further explanation necessary.
If you know someone who is going, who doesn't drink, hang around with that person as much as possible. Concentrate on the conversation, the music, the food. It will feel weird. You will survive it.