Well, you already have 3 convertibles, and since you need one for ffing, I'd probably buy a 5pt harness to booster seat. They generally have a taller shell so they'll last ffing until DC is ready for a booster. Look into the Evenflo Maestro, Graco Nautilus, Recaro ProSport, or Britax Frontier.
I'd keep one of your Britax seats rfing in the nanny's car. I think the Radian might be too hard for her to tighten correctly, especially rfing.