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...

I agree with the other comments, wait until you white wash, but I would be tempted to paint some colour of the walls if you cannot remove the trim (it might be there to tidy up the joins).

Hi Lucy Have you thought of using the same colour on the skirting boards and architraves but doing them in high gloss? That way they will look slightly lighter than the wall due to the light reflection off the gloss.

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.