I read your post with interest,
Varnafinde, especially as since you last posted, three things have happened. We paid another visit to the Tower of London and Buckingham Palace in September last year. At Buckingham Palace there was an exhibition of the Queens tiaras some of which she wears on Australian coins etc. The second thing to happen was the abdication of Queen Beatrix of the Netherlands, and the inauguration of her son, Willem Alexander as the new king about 1st May 2013. And the third was the 60th anniversary of Queen Elizabeth II's coronation in June. When the original event happened C.S.Lewis was in the middle of publishing his Narnia series, and I am not at all sure that George VI's death and the coronation did not have significant influence on his Narnia series.
Though I agree that the king of Norway has dispensed with fancy coronations, unlike the British, I'd have to say he is no Robinson Crusoe in post-WW2 Europe. The Dutch inauguration was quite unexpectedly casual, despite the balls, banquets and formal dress of crown princes, and I don't think any of the other European monarchies have anything so formal as Queen Elizabeth II had 2 June 1953. That was a religious service which very much pushed the idea that Queen Elizabeth II was to be queen for life. Just as the Pevensies were told "once a king or queen of Narnia, always a king or queen of Narnia". I've some other observations to make in general.
1.There are some occasions when a noble would wear his/her coronet other than at a coronation. Charles, the Prince of Wales, had to get a new coronet made when he was appointed to that position by his mother because the previous Prince of Wales had run off with it. By implication, when someone succeeds to a title they might get to wear the coronet when the appointment is made official.
2. It is true that Queen Elizabeth II only wears the Imperial State Crown for the opening of each session of the UK parliament, but the crown used to crown most British monarchs is the St Edward's crown, which is unused otherwise. It was dusted down and given an airing for the Coronation anniversary church service. These even look very heavy things to wear, and last May our 87 year old Queen looked quite bowed down by its weight when opening Parliament. The Imperial State Crown is nowhere near as old as others in European museums. Oliver Cromwell's Roundheads demolished much of the previous UK crown jewels, though the Honours of Scotland still remain.
No wonder then that Pauline Baynes would have had no difficulty in drawing Narnian kings wearing in all sorts of situations, the much lighter, more wearable, even helmet-like crowns, in use before Oliver Cromwell. A case in point is King Lune wearing his crown whilst out hunting, and Prince Cor wearing a circlet to visit Aravis at the Hermit's place.
3. Tiaras get worn a good deal more frequently, as you remarked. Many of them have been around for a fair while, despite wars, revolutions, pillage and the dethroning of original owners. Sometimes these tiaras can be taken to pieces to form necklaces and other jewellery. One favourite tiara the Queen often wears is
this one in which the emerald drops can be interchanged with pearl ones.
4. Most nobility have their own family tiaras, which are worn for weddings as well as other formal occasions, such as the Duke of Northumberland's daughter Melissa, who a week or so ago was the bride at Alnwick castle. Diana, Princess of Wales also had a family tiara, since her father, and now her brother, was Earl of Spencer. The Queen seems to have a tiara lending service which the Countess of Wessex and Duchesses of Cambridge and Cornwall have all used. At the Dutch abdication and inauguration all the crown princesses were wearing
a range of tiarasregardless of whether they were consorts or heirs to the throne in their own right. This included not only Crown Princess Mette-Marit of Norway, who looked really nice, but also the Crown Princess of Brunei who wore a tall tiara over her hijab.
I'd imagine that tiaras would be worn frequently by Narnian and other Queens, especially as I would think they would be much more imaginative, decorative and artistic. For a movie you wouldn't need real jewellery anyway.
5. Lastly, the Queen is never seen publicly without a hat, or at least a scarf. And there are rules about others wearing hats on formal occasions. They have to ensure that their hats are smaller, or left off altogether. There is another reason why Narnian humans would wear hats, even without a crown - the weather is colder in Narnia in winter, just like in UK & Norway.