Ok. As I suspected and makes sense now.
Substituted service is not a battleground, even if they don’t mail. In SC, they could have served you by certified mail. If you show-up and the court accepts that argument (and I don’t see that as a cause of delay), you’ll just be considered as now having notice and you’ll either get the opportunity to proceed that day (these aren’t complicate matters) or you’ll be given the option of some new date in the fairly immediate future. A discovery issue might justify a bit more time, but it will happen, so go with the flow and file an a response, so that you have a right to get documents. Don't bother to battle over service.
As to the earlier remark that there was no discovery in Small Claims, that’s true in the general sense, but there is all that you need. Look at form SC-107, a fillable form that requests documents and you can find at the CA online self-help site. If your hearing date is in Jan., you have time to use it if you act promptly. If you do, check in the Debt Collection forum, if you don’t already learn what to request. If you serve and don't get a response, tell the court. That's why it wise to serve via certified mail.
Discovery aside, if the debt is yours and not time-barred, the result is probably a foregone conclusion. If you haven’t spoken to the credit union since you were served, that’s the entity to speak to for a negotiation. At this point, no other entity is involved with authority to negotiate. If they’re going forward themselves, they’ve taken it back from the agency.
If it must go forward and it’s “nothing ventured, nothing gained”, defend yourself. In addition to any other defense you may discover, it never hurts to challenge the witness’s capacity to testify. The witness must be “percipient”. That’s such a common and assumed capacity that it’s seldom questioned. It means the witness must have full personal knowledge supporting the validity of the evidence being presented or have the same in a Declaration under oath. It means that, without a Declaration, the witness must have personal knowledge of every act that went into the preparation of a bill: the basis of the debt, any prior notice to you and everything else about this. If this was from almost 4 years ago, it’s probable that only the top and long-time people in accounting and collections have that. If the debt was from 4 years ago and the witness is a junior employee employed 2 years, he/she can’t. Ask the questions in court and object to introduction of the evidence. Case dismissed.
If they’re determined, they will re-file and fix the problem, but the longer they go and the higher-up they have to go for testimony, the likelier you get a chance to negotiate. You don’t need a credit record with a judgment, even if paid.
Good luck