Reading through all this really makes my head ache. I agree with Stymie and so here are 2 scenarios that illustrate my view on the matter.
S=Slayer G=Goblin B=Bigboss
the letters don't line up perfectly :S.
So if the bigboss goes 1st and kills 5 guys then I would give the models in contact with him 5 attacks to hit where they want. There are 3 models so I would split the attacks among those 3 as evenly as possible randomizing the extra one(s). The goblins go next and kill 10, they are in contact with 6 and so i would again split them among those 6 as evenly as possible. So in this case if the dwarf player wanted to kill the bigboss he could direct at least 2, most likely 3 or 4, attacks on him. The case could be made that the dwarf player chooses the position of the extra attacks like with shooting at a unit less than 5 in numbers but the counter argument could be that the goblins can choose who to hit, so random is the best imo.
S=Slayer Z=zombie or whatever W=Wight King or some other character V=Vampire or some other character.
V goes 1st and kills 6 guys, here I would say that since he is only in contact with one slayer the deathblows also come from that one spot, all 6 of them. These 6 attacks can be directed against Z, W or V any of those since the model being struck is in contact with all of those. So the dwarf player would be free to split his attacks any way between those 3 or hit all 6 on the wight king possibly killing him before he swings with his Initiative which is lower than the vampire but higher than the slayers.
As for RNF attacks from one unit to another I think they should be split as evenly as possible among all models on base contact with the attackers.
Edited by Kafki, 13 April 2014 - 12:01 AM.