The duty solicitor is fine, but it would be better to get the services of a solicitor before you get to court rather than instructing one on the day. As to the charge, the CPS can have you charged with anything which is disclosed on the papers before them. That may mean that they decide on a charge which is more serious than the offence on which you were arrested, obviously, but also one more serious than that which they originally directed. Charges do get reviewed sometimes .This is common when further evidence emerges, or where the case is serious, difficult, or complicated and the CPS has taken advice from counsel who has advised that other charges be laid.
You have been lucky so far. This looks to be clearly a case of affray and you are fortunate that it's not a s18 (gbh intending gbh)