Hi Neil,
This how I would play it:
a) Just one element it says, so not more than that. Because the scouting player can't choose his opponent has to choose one of his most forward elements in ambush.
b) You allocate the scouting elements simultaneously. I you role at least one 5 or 6 with them all hidden elements are discovered, if you don't but at least role one 4 the most forward element (just one) in ambush is discovered.
By having more scouts you upgrade the change to discover the enemy at the greater chance to loose some of them.
Greetings,
Marcel