Again, I don't see anybody suggesting that. It is true however that you can study a country all you like but living there is a very different kettle of fish, especially in the unique role Meghan will have to play nationally. I don't doubt that she'll do it very well indeed but it'll be a learning curve. The British people are fickle types and sometimes surprise people. For example, the Americans tend to be encouraging and celebrate their achievements openly, the British don't and tend to be more subdued. And just as someone figures that out, we change the rules and boast a little leaving the person totally perplexed. Thank goodness she'll never have to master a revolving door in the UK when we apologise to the person in front, the person behind, ourselves and the door.
This is where I absolutely agree with you. The Royals don't self promote. They have no need to. As you rightly say, what they do is promote a brand - that brand being the United Kingdom. There's only room for one star of the show and that's the Queen. Every member of the Royal Family promotes their own causes and builds unique relationships with charities and organisations that last a lifetime. But at all times, there's absolutely no time for individual successes or praise. Look at Meghan's future father-in-law. It's taken years for anyone to say "Well done" on his environmental work or even the Prince's Trust. And even then, he'd never have expected such praise. Meghan might find that approach hard to adjust to at first. Being a celebrity does require a little self-promotion outside of promoting a recent TV show or movie so I think she has had to push herself forward in the past. But I also don't get the impression this was a big deal to her. I think she seems to have enjoyed her work more than the limited fame that came with it. So as you say, her skills will be an asset.
But I think what Denville was saying (and what I agreed with) was that the Royal Family are never ever celebrities. They might appear globally in gossip columns next to and with celebrities but they are not celebrities. Precisely for the reason you mention: they're working for others, not themselves. That's the distinction. Being a minor celeb will help Meghan prepare for her new life I've no doubt but it ends now. She is no longer a celebrity. She's famous and always will be. But she will now be a member of the British Royal Family and that comes with a whole different rule book. Celebrities share alot of themselves, they covet fame and success, they seek rewards. Royalty doesn't. It's when people see elements of celebrity creep in (It's a Royal Knockout, Fergie on QVC) that they panic a little. The institution only works when it's at the top of the food chain. Anyone can become a celebrity, hardly anyone becomes royalty. If anyone could have what they have and do what they do? We'd be a republic and we wouldn't be chatting about the future Princess Henry of Wales at all.