I don't think any witness on this case had enough info to cop any deal.
Wait, I take that back, RN had enough information to get a plea bargain.
But RN's information supports one of their charges and she was actually involved.
I do think it is possible that the prosecution made an agreement not to prosecute someone for drugs if they gave information about Billy being a drug runner.
I think someone spilled the beans on that.
I'm not sure of the definition of witness as we speak of it here.
Anyway, your second point is a real possibility.