Meh, knowing the rules to break them only goes so far. The ones usually touting that usually have some rules to sell you. Brilliant move with the California top there quirky, coatigan and jeans look good too.
I generally ignore hourglass advice, mostly because it's wrong as hourglass often gets interchanged with curvy. But, WRT checks, I've mentioned it before, it distorts me, I turn into one of those magic eye posters and it's not really about looking heavier or shorter or whatever, it's asymmetric issues, like one boob looking noticeably bigger than the other, or one buttock, it even distorts posture sometimes, it's probably not that noticeable to others but it constantly distracts me and like it or not evolutionarily speaking asymmetry in the body doesn't read well. I had hoped that perfectly pattern matching things would mean I could break the rules but I think I need a diamond pattern to make it work for me.
I agree that the flatter and dress to your shape mantras can be too much like a straight jacket, but as much as that makes me feel all sorts of claustrophobic, I get the sense a lot of people enjoy the rigidness of it, ditto getting your colours done. Obviously there's truth in it all but I think it should be treated as guidance as opposed to rules. I'm not that audience that just wants boxes to tick off though and get dressed with this supposed universal knowledge of good V bad taste, I enjoy talking at ridiculous length and experimenting with disastrous result but can't get around the basics like proportion and colour. I accept that will always be a niche occupation of time wasting that not everyone wants to engage in.
I think that dressing with distance goes for most British Royals though, except unlike MM the boundaries are fairly clear and it reads very much as in function wear. I think MM stood out because she was trying to be both elevated and also personable and it ended up reading as a bit bland and full of not quite successful references to Jackie Kennedy, Grace Kelly, French Vogue editors, etc. and therefore little or any of herself and more like an overstuffed Pinterest board. I can't really say if Diana ever managed that personable/elevated balance in her style as I just don't remember (I actually remember her as a bit of a questionable dresser but I was probably too young) but it seems to be an accepted truth now, but I think that was sort of the goal. Granted I didn't really pay much attention to her much, except whenever something trickled into mainstream news here, which wasn't often, so most of what I've seen of her would have been her early royal life and the departure but it all read as if she was a bit lost and out of her depth but that could very well be me subconsciously marrying her struggles in Royal life with her outfits with hindsight but fairly sure I thought it at the time too.