Leave it down 25mm or so and fill gap with mineral wool, this ticks thermal and acoustics. If its fire resistant (infill walls usually are not unless you have an inset balcony or right up close to a boundary line) you can just put in a steel angle to the inside or rely on the mineral wool...
You will generally only get ~15mins performance completely unprotected, at least under an ISO Fire Curve.
You can get up to 60 mins with partially exposed beams/columns (Beams with shelf angles/columns with block infill etc.).
It is however far more starightforward to apply intumescent paint...
Sounds like that cladding system was designed for an Aluminium Million rather than CFS. You can technically do it the way you describing however meeting the deflection will likely be a struggle as you your deflection length is now also between studs? So ~400mm in length (like 1.6mm of a limit)...
