In answer to your Q1... yes, ignore DCB Legals's request to discuss settlement.
Regarding your points about the latest N180 DQ, it is only mandatory to "attend" the mediation phone call. It is not part of the actual judicial process. There is no judge or solicitor involved. It is a waste of everybody's time in these types of clams.
As you are already aware, you offer £0 and that is the end of the matter. The claim progresses.
Regarding your Q2... as long as you defended the claim, there is a greater than 99% chance that DCB Legal will discontinue before they have to pay the trial fee (£27). They rely on the defendant to be low-hanging fruit on the gullible tree who is going to capitulate and pay up out of ignorance and fear. You are obviously neither ignorant nor fearful of the process.
However, the extremely unlikely event that this ever progressed as far as an actual hearing in front of a judge, you could represent your father as a Lay Representative. You would only need to tell the clerk when you arrive for the hearing that you will be the defendants Lay Representative and make sure you take a copy of the
The Lay Representatives (Rights of Audience) Order 1999 to wave in their face if they or the judge questions this.
As a Lay Representative, you may exercise rights of audience in proceedings dealt with as a small claim in accordance with rules of court.
As a Lay Representative may not exercise any right of audience where the Defendant does not attend the hearing. So, your father would need to be present and may have to answer any questions that the judge would direct their way. However, you would be able to confer with him.
As a Lay Rep, you cannot be a witness or give evidence, only represent the defendant.