From the descriptions it seems much more possible that F 345 was sunk by the british MTBs than by the US-PT-boats (because the latter's victim beached itself and F 345 sank).
...The German records do not mention the British MTB 298 at all...
Why not? The german source says that the F-lighters were attacked by "S-Boote", the german term for small fast boats carrying torpedoes as the main weapon.
Of course they were not aware of the number/name of the boat.
And further the german war diary says (Darius did not mention this) that there was speculation about the nationality of the enemy. Italian MAS-boats were more or less excluded because of their range and british forces were thougt possible.