We have established that even if you slay a challenger, you are still considered to be in base contact with that model until the end of the phase in which it is slain, per the Challenge rules.
I do not think this is true - the rule states that, For the duration of a challenge, the two combatants are considered to be in base to base contact with only each other.
That is not the same as saying "For the duration of a challenge, the two combatants are considered to be in base to base contact" or ""For the duration of a challenge, the two combatants are considered to always be in base to base contact with each other"
The challengers are not always in base contact, they are just never in base contact with anything else - thus, if one challenger dies the other is no longer in base contact with anyone else and wounds are allocated following the "nearest model" rules.
The fact that there is a "wound sponge" for the duration of the challenge is predicated on the fact that the two challengers are always in base to base contact, which is a grammatical misinterpretation of what is written in the rules.
There is a huge debate on this on practically every 40k forum currently... we are lucky enough to have Sam to make a decision we all agree to until the next FAQ comes out in August (and hopefully addresses this).
Look to the first paragraph under Challenge. It explicitly states that the models must be positioned in base to base combat, and if you cannot, then you *assume* that they are in base to base. They are in base to base the moment the challenge is issued until one is dead.
"If a challenge has been accepted, it is time to
move the two combatants into base contact with each other. Note that these moves cannot be used to move a character out of unit coherency. If possible,
swap the challenger for a friendly model in base contact with the challengee. If this cannot be done,
swap the challengee for a friendly model in base contract with the challenger. If neither of these moves would result in the two models being in
base contact 'swap' the challenger to as close as possible to the challengee and assume the two
to be in base contact for the purposes of the ensuing fight....
for the duration of the challenge, these two models are considered to be in base contact only with each other.
Show me how that they are not always in base contact, and only with each other.