Hi Brooke If then plan is to fully renovate then go for it...! It will be just fine. A new finish on the floor will lift it too. Doing the floor before the skirting boards will save skirting getting damaged and having to touch up paint. You can find a white to go with your grey by reducing the grey down untill its white then you won't have clashing undertones. Other wise check with your paint store for the whites to compliment your grey choice. :)

have had faded mission brown aluminium windows and wish i'd thought to paint the architraves on the big windows and sliding doors a very dark colour to frame the view but prefer very dark brown or charcoal and match the smaller windows to wall and doors...Why not tape some strips of dark coloured paper over the architraves to have a trial of how it could look and the undercoats can be tinted for various affects before the final coat?

Typically the "shadow line" detail that your builder is referring to is a plaster finishing detail, commonly referred to/known as in the industry as a "P50" which is particular plaster edging/stopping profile that creates a 10mm shadow between the edge of the plaster and whatever it is abutting - it's a detail inherited from commercial construction, and is usually a more expensive detail to achieve if done properly. Also unless it's managed carefully from a design point of view, and committed to throughout a project, across joinery and other aspects, it can actually achieve more of "bland" effect and I'd argue that shadowline is an incongruent detail to a traditional renovation/extension project, unless you are clearly articulating the old and the new, have a super slick architectural design to the project, and/or you'd prefer the "hospital room" vibe

In your example I’d paint the trims the darker colour but have the whole room the darker colour not just a feature wall.