Afaik, the new leader will automatically be assigned by ofp, and he will therefor have acces to the radio command.
But what I understand from your first post is you want every group member to be able to call the artillery.
I assume you have 4 players in your MP mission, p1 p2 p3 and p4.
Once you reach the final stage you enable the action for all players. In your last part script add these lines:
artillerystrike = p1 addaction["Call Artillery", "artillery.sqs"]
artillerystrike = p2 addaction["Call Artillery", "artillery.sqs"]
artillerystrike = p3 addaction["Call Artillery", "artillery.sqs"]
artillerystrike = p4 addaction["Call Artillery", "artillery.sqs"]
this will give all units the option to call the artillery.
Now don't forget to remove the action once one player has requested the artillery.
eg. in artillery.sqs make the first line:
p1 removeaction artillerystrike
p2 removeaction artillerystrike
p3 removeaction artillerystrike
p4 removeaction artillerystrike
Does this make sense to you?