As I've got my rulebook out now

The 'Scouts' rule is different from the 'Infiltrators' rule, they just both as a bonus provide the 'outflank' ability when in reserve
A scout move is a 'normal move' taken before the game begins, in which case is up to 24 inches from a 'fast skimmer' (18 inches for a fast vehicle). The only restriction, is that it must end up its move more than 12 inches away from an enemy unit.
The 'flat out' speed is defined as more than 12" and up to 18" (24 inches for a skimmer that is also fast and is moving flat out).
So, from my understanding,
Combat: 0 to 6
Cruising: 6 to 12
Fast Flat Out: 12 to 18
Fast skimmer Flat Out: 12 to 18
There is a restriction that passengers may not disembark from a vehicle moving at flat out in that movement phase.
So, it is entirely possible for a valkyrie (which doesn't have firepoints) to move flat out in the scout phase, move forward in the movement phase up to 12 inches, drop off a squad, particularly meltas, have them fire, then it fire its multilaser (main weapon), then rocket pods (defensive weapons). Of course, assaulting isn't particularly an issue for me

And yup, your deffkoptas seem to do that quite happily. However, if you have more than 1 with a buzzsaw, they'll end up costing more points than the manticore they're trying to kill. And even then it isn't a 'guarenteed' kill. Highly likely, but nowhere near certain

And that's good enough for me.