For now from what I hear in Brazil the new conilon crop is expected to be record, so I wouldn’t be to worried about exports availability and differentials at the beginning of the new crop.
I think in the short term spreads and risks of squeeze for robusta on the terminal market is the biggest risk. The is no production shortage in robusta, it’s a logistical problem at the moment, so if the market rises it should be bearish for differentials (FOB).

Regarding reduced export availability if the arabica crop is really too low, it might happen but these things takes time. First the market will need to for push more usage of conilons / less arabica in the local Brazilian market to free up export capacity in Arabica.