You're quite right Mad.
Pg 74 BBB. Fleet of Foot is marked with an asterisk, denoting a special rule that is lost by an independent character who joins a unit that does not have that skill. Incubi cannot
FoF, so the Dracon may not as long as the retinue survives.
Pg 15 BBB. "
All models in a unit move at the speed of the slowest model."
Pg 37 BBB. "Start the charge by moving a single model from the charging unit.
The model selected must be the one which can reach the enemy by using the least amount of its available movement. Move the model into contact with the nearest enemy model in theunit being charged, using the shortest possible route."
Pg 38 BBB. "
All the models in a charging unit make their Assault move subject to the same rules as in the Movement phase, but may be moved within 1" of enemy models they are charging."
Consequently, during the assault phase, you are limited to the movement of the incubi (speed of slowest model) and cannot complete the assault if none of the incubi can make it into base to base as per the above cited rule.
Sal.
PS. If it were a
GW store, I'm not surprised they couldn't find the ruling