To clarify a bit...
Support fire can
only be allocated to units in range of the assaulting formation.
It doesn't matter if you have 100 units from support formations and the defender formation within 15cm range of each other. Shots are only fired at the defender units which are directly involved in the assault.
Quote:
After the charge move (in this case an Air Assault) a Vulcan Dreadnought was within 15cm of enemy units. After allocating non-MW hits and making armour saves it was now time to roll for hits for MW-attacks. But now all enemy units within 15cm of the Vulcan Dreadnought were destroyed from non-MW hits. So could the Vulcan Dreadnought still use his FF-MW attack from it's Multi-melta or not?
First, this situation would never occur. You have a misunderstanding of the sequence of play.
MW attacks are rolled at the same time as regular attacks. Range is determined at that time. It's not possible for a regular hit to blast a target away and then prevent the MW attack due to lack of targets. The MW hit is scored before any hits are allocated or saved, when all targets are still present.
MW hits are
allocated after regular hits have been resolved.
Second, you don't track which unit scored which hit for purposes of allocation. The only requirements for hit allocation is range and line of sight and range from
any unit in the formation. There are no distinctions between which unit and which weapon are used to qualify for LoS or Range. The source of the original hit is irrelevant.
So, even if all the units in range from the Dread died, it doesn't matter. As long as any enemy unit is in range and LoS of any of your units, the hit can be allocated to that unit.
Yes, this can make for some odd results, like an attack from a Dread hitting something on the opposite end of the assault, possibly quite far away. However, it keeps players from having to track which units scored which hits in order to know which enemy units can be allocated hits. It also keeps people from doing "gamey" things like allocating all the hits to the units around a Dreadnought so that when they all die, the MW hits are wasted.