I have a conventional concave cornice and I get plenty of cobwebs in the 10mm step out from the wall plaster (but not so much the step out from the ceiling plaster, or maybe it doesn't show so much there). If you are replacing the ceiling I would go with a shadowline rather than square-set which will show every wobble in your plaster or paint. I don't think it is feasible to install a shadowline into an existing ceiling (which will usually have a ragged edge under the existing cornice) but I could be wrong...

You'll need to forgo goggle and take yourself into the closest wood supplier. The skirts and archs look fairly common, similar to mine in fact. My local wood yard also a Home Hardware had them.

Those cornices are both gorgeous and look to be in excellent condition. When extending homes that have some original character and features, there is no need to gut the original rooms. Unless you would like to pretend that the original home never existed and that you have actually built an entirely new home, keeping the original rooms as intact as possible and then, working with your architect and builder to transition into the new rooms in the extension can often make for a more beautiful and original renovation/extension. often the more ‘different’ the two are, the better. Ie, ornate cornice and skirting with original polished boards transitioning to square set and polished concrete for example. if you are wanting an entire new and modern building then buying a beautiful old home would seem an odd choice.

Seriously? Lack of understanding of the authentic original period architecture. This is neither fish nor fowl and so overcooked. Less is more people. The best results in renovations/refit scenarios are always those that are inherently sympathetic, taking cues from the existing space or original style, and tying in. It can be successful taken either in a contemporary direction or restoring accurately, BUT regardless of what it is it needs to be BALANCED, rather than the mess like here which is a case of arbitrarily applying an excess of random foreign design and style elements. People (including interior pros) need to stop trying to be "on trend" for the sake of it or thinking we must have "features' or 'hero' elements. Sometimes the best and most clever architectural results are those that promote LESS attention to it......but enduring and classic outcomes requires subtlety and capacity to confidently filter stylistic choices without being influenced by the noise of the latest interiors magazine or Pinterest post.