It will almost certainly just be you and the adjudicator, it's really rare for anyone from the Council to turn up, doubly so since the adjudicator is likely to ask them some embarrassing questions about the irrelevant 50cm nonsense.
Don't expect the adjudicator to want to hear everything you have to say, they are quite likely to stop the proceedings once they decide you have a winning argument.
I think the quickest route would be to start by asking the Adjudicator if the council are allowed to change their mind about which offence they issued the PCN for. The answer to this is of course, no. You'll probably get half way into pointing out that they changed their mind from Red Route to 50cm before the adjudicator makes their mind up to write some rather choice words about the Council while ruling in your favour.
In the unlikely event that the adjudicator wants to discuss you being more than 50cm from the edge of the carriageway, don't panic, you're actually in a situation where you could admit you were guilty of that and not be in in any danger of paying a fine for it, because there's a time limit on making accusations and it's too late for the Council (or the adjudicator) to go there (among other reasons). The line to take is that you would have considered your options if you were issued a PCN for that, but that's not what happened. You're here to dispute the PCN you did receive, not a theoretical one that the Council might have issued.
In the even more unlikely event that that approach doesn't win the case for you, move on to asking the adjudicator if the Council are required to consider your representations? If so, does a whole load of irrelevant waffle about being 50cm fomr the edge of the carriageway prove that they did so? The answer to these questions are going to be "yes" and "no".
In the unlikely event that the adjudicator has still not ruled in your favour at this point, move on to your initial argument about this location not being signposted as a red route, and ask if the Council are allowed to withhold evidence about this until the Evidence Pack stage, despite you asking for it at the first opportunity.