They are right that RFS only applies to standalone bookings. If you have the 5th freedom flights as part of a long haul itinerary you will pay the full taxes and surcharges. Them's the rules.

They are wrong about the avios amount for your domestic connections - these are still free on long hauls. A dummy booking on BA.com from NCL-somewhere and LON-somewhere will show this. The taxes and surcharges will be higher though because again the RFS cap won't apply.

So the £200 extra is probably right. The 4500 avios extra isn't.

ETA: Avios awards in Y still include generous checked luggage so why not just book the intra-Caribbean flights separately? How many flip flops and swimshorts are you intending to take?!