You don't need two resources, just different Cost rate tables.

I'm assuming there are some sort of tasks the Jon works on Thursday and Friday which are somehow intrinsically different than the Monday through Wednesday tasks? You can assign different cost rate tables to assignments. However, the issue will be if a task starts on Monday and extends to Thursday. The cost rate table applies to the entire span of the task - not just part.

I also do not know of any method to say that if the duration is longer than 3 days the entire task should cost nothing? I'm afraid it's going to be piece by piece.

You can work with fixed costs or cost resources instead of costing at the work resource level - but this still is going to be very manual.

Can you explain your scenario a but further? I've personally never been in a situation where someone "volunteers" for two days out of a week.