We did the Northern Europe itinerary in October half term with our 2 children. I can't answer all your questions because my wife organises everything and I just turn up 😄
Generally, other than the theatre show, you can show up at any of the venues at any time, although finding a seat might be a challenge, we didn't have a problem, we managed 3 shows some nights going from the Clubhouse to theatre to Skydome.
With the theatre we did book in advance, but if it wasn't full 10 minutes before the start time they will let you in.
There's one Celebration per week on Iona and I would say the majority dressed up, I don't do black tie myself, just a business suit but plenty gents did do.
Restaurants are a matter of what food you prefer. We enjoyed Sindhu which is contemporary Asian food. Keel and Cow do good gastropub meals, and is good value. Don't overlook Olive Grove, most meals are included in your fare but there are some items available at a supplement.
If you want something more laid back, but a step up from the buffet, the Quays is good, quality fish and chips, American Diner food and curries, more like a food court.