In the case you are describing, the Musto may very well have rights if there is an overlap between the two boats when the F18 reaches the zone.

If no overlap exists, then you are correct in saying that the F18 does not owe room even if later an overlap is established.

It is also important to note that the penalty would be a two turns penalty (360 and 720 are no longer in the rules), unless the SIs amend the penalty to one turn.

I also don't understand the anti-dinghy sailor sentiment. I sail both cats and dinghies- there is zero difference.