I shall use my limited expertise on weddings to help you out!
Do we need to stand at the door shaking hands with everyone?
No, not necessarily. You can treat it almost like a church, groom welcomes everyone in with the help of best-man and whatever and then everyone finds their seats, waiting for the lovely bride to arrive. However, it's up to you!
Do we get everyone to LOOK at us while we cut the cake?
What seems to work is the master of ceremonies or best-man just asks anyone who has a camera and wants to take a pic of the cutting then feel free to come forward and take a pic or just watch. Some get up, some stay seated.
What is all this first dance stuff?
Whatever you want it to be, you don't even have to do one! Most couples just choose a song they both really like, and have a dance while everyone looks on with misty eyes. In pukka formal events, it's up to the best-man to decide when everyone else can join in dancing by joining in also, but that's not absolutely necessary.
Do we need to introduce people to each other?
Not at all, people can do that themselves. Of course if you're in a conversation with people who don't know each other, it would be courteous to introduce them to each other, but introducing EVERYONE isn't needed.
If things kick off, are we meant to intervene, or just hide and hope weapons don't get involved?
Erm, I've never seen this happen before so I'm afraid I wouldn't be able to comment from experience!!
What about presents? Do we open them when people give them and make a big fuss, or put them in a big pile to open later?
Just leave an empty table at the entrance for people to leave presents and cards, and then collect them at the end of the night. Probably best to open at home!
Do we need to announce things like the buffet?
Either get the master of ceremonies or best-man to do this.
Am I meant to dance with my FIL or something? How about if there is only rock music?
Only if you want to! And it's up to you to employ a DJ or band, and most will talk to you about what music you want.
Are we meant to give everyone a cake and a balloon to take home?
Hahahaha, very good! Some function rooms I've been to take the cake away after you've ceremoniously cut it and then place it (all cut up into small pieces) at the end of the buffet table for people to help themselves.
All in all, remember it's YOUR day, so if you do or don't want something doing, then organise it that way. Most of all, congratulations!