This is really good advice! Kicking people out of the event is necessary especially at larger venues.
I'd like to add a few things I observed during the past three weddings or so that I did in the game:
1. Use small venues. Guests won't wander around as much as they do with big venues, and they actually know where they are supposed to be. I tested this with a small venue I built for a vampire wedding in Forgotten Hollow. The wedding arch and aisle are outside in the garden, the building is on top of a small hill nearby. Every single guest stayed put during the ceremony; not a single one wandered up that hill and into the building. They only did so when the newlyweds led the way. That vampire wedding was actually the most perfect one I ever had. The only thing that went wrong was the cake being spoilt because they bought it too early. But other than that - perfect.
2. Group guests with the couple. It's become the first thing I do once everybody shows up at the venue. Grouping them ensures they go whereever the couple goes. It's not always working flawlessly, but it usually gets the job done.
3. Timing and the correct order for the ceremony are essential. For example, during my last wedding, the "walking down the aisle"-bit worked almost perfectly for the flower girl, the witness and the couple itself. The officiant was a bit too slow (she's an elder) and was not directly under the arch when she performed the ceremony but slightly to the side. Pausing the game to get the order right is super helpful because you can queue up everything in the right order, including the prompt for the guests to sit down. This should ensure they stay on their seats during the actual ceremony.
4. Don't host outdoor weddings during autumn or winter even at venues that are in a world with a mild climate - I know, it's a no-brainer, but I forgot about it during the last wedding, and of course everybody changed into their winter clothes once they were outside (the ceremony took place inside). Also, expect guests to do the same after a vow renewal. Those are much shorter than a "real" wedding and guests tend to change into their everyday outfits shortly after the ceremony. Also, don't plan too many activities for a vow renewal. Otherwise, your couple might end up celebrating alone because everybody else has already left.
5. Make sure the bridal couple has potions for hygiene and energy if they didn't get enough sleep or didn't have the time to shower properly - this is especially useful for impromptu weddings because it makes sure the couple doesn't lie down on the next bench or sofa to take a nap, and they'll stay fresh, too.
The clothes thing is something that really annoys me. I mean - I get why guests and couple change into warm clothes if they have to celebrate outside. Perfectly normal. But more often than not, Sims I created show up in random outfits I never picked for them, and setting a dress code when planning the wedding doesn't always work, either.
