I'd hope that if ducting has been recently cleared for fibre, that they'd have run a lot of spare sub-ducts (essentially plastic tubes), so that they can blow more fibre through without having to clear the ducts again.
Believe the ducting roll-out is scaled to serve the cabinets from the aggregation node and also any subsequent FTTP build, either FTTPoDemand, or native FTTP if the decision is taken in the future, or to support FTTrN. Use of GPON for FTTP reduces fibre count
Only if the sub ducts were sealed. I am assuming as it is possibly to pull additional fibre through them that they aren't sealed in which case there most be places where silt and other blockages can enter from?