Our walls are Dulux off white with a white ceiling. I’d go semi-gloss to capture and reflect any small amounts of light.

Pretty sure ti tree and eucalyptus oil wiped around the place will make a difference. Wipe surfaces such as skirting boards and door frames etc. Good on you for minimising chemical exposure.

My hallway is dark and needs a paint job. I was told not to have my cornices a different colour to the wall so it gives the appearance of height. I’m going for white walls, cornices and ceiling.

You just described my hallway! We’re going with sandy day half (half being half the strength of the full colour), antique white is very popular at the moment both are dulux colours. A good painter could recommend something, most people go for neutrals these days