To dam ugly,
you ask some good questions but I think you have one wrong.
Your question b. should be: did the producer who paid the bills accept the risk of hiv being transmitted on his set and what did he do to prevent it? The simple answer is yes he knew the rilk, and did nothing to prevent it.He provided no type of protection on his set.

Your questions about temps versus contractor versus emplyee are very legit also. Thats what courts are for i guess.

What I think the outcomje will be, and this is only my opinion, is the producer will be liable for what happened on HIS set.He was paying the bills. He was the one who set all of these things in motion. If you throw a rock into the air that is not a crime. But if that rock hits something and breaks it you are responsibel because you started the chain of events that lead to the damage.Just my opinion.