The Head of Norland owns the school and is unlikely to leave anytime soon. They would be a very good option and are a nice and nurturing school, albeit perhaps less likely to offer a sure-fire path to the most competitive day schools.
Personally, for a child who is strong but perhaps not quite SPGS-bound (although who knows, things change and children develop a lot at that age!) think it will be even more important to be in as strong a prep as possible in order to have the best chances to get into a decent public school. Many children increasingly have to board despite parents not necessarily wanting them to as it is so tough getting into London day schools these days.
As a previous poster had mentioned, perhaps your older daughter can commute until a space at a better prep opens up for your younger one? There may well be some leavers over the Summer and beyond, once people find those country houses which seem to be in such short supply at the moment.
Bute would be best in terms of location and options further along, but is fierce to get into and unlikely to have a place now that the 7+ is done. Falkner House is top in Ken/South Ken but equally fierce, Glendower a bit easier but not by very much. I wouldn’t go for Queens Gate if you have higher ambitions for public school, but on the upside the school goes through to A-levels and that may put your mind at rest.
If you’re intent on living in Kensington worth looking at Thomas’s Kensington? Lovely school, very local, well rounded and also goes through to A-levels now that they have the senior school, albeit that’s in Battersea. Same goes for Thomas’s Fulham. They must have a bus to the senior school, when the time comes.
Good luck!