Chris,
Braking is permitted at main wheel touchdown. At high nose attitudes there is still lots of lift even with spoiler so not much weight on the wheels. Hence higher pitch longer landing distance.
Speed increment remains the same because the slats are not increasing CL and the higher pitch for same Alpha doesn't help adding even more pitch for a bit more alpha (Cl) is counter productive as it extends the landing run.
Extra Slat is neither inhibited nor prohibited but the data is the data and the landings will be longer.
Clearly the aircraft isn't geometrically limited at slat (123) as the aircraft can land with slat (45full) and yet the Vref remains the same and the landing distance correction is considerably greater than with no Slat.
I can't really add anymore as I'm just recalling what Emb told me when I asked the question.